Why were archers the primary force in Achaemenid armies?

[Blatta Optima Maxima]

Civil rebellions? Those were extremely rare. Most of the time it was ethnic Persian satraps rebelling. The troops weren't provided by the provinces, they were provided by the Persian rulers of those provinces. Those troops were raised through feudal contract or paid, and were not more or less reliable than Persians by default, since they weren't "national contingents" as sometimes perceived.

[Kaveh]

Yes civil rebellions were very rare indeed since there were no need for them.
Keep in mind the fact that around 50% of the world population were if not content perhaps elated that they were under Persian rule because they were allowed to keep their language,cultural,religious,local identity due to the tolerant policies of the empire and in many case their cultural achievements flourishing better than ever before due to the economic support provided to them equally through out the estimated 30 Satraps across the Empire.

This obviously ended when Alexander conquered the unified empire and decided to set out to make a Hellenistic world empire where his Capital would naturally be based in Babylon and reign over the entire world being ruled from a variant civilized world to a Hellenistic one.

But at the peak of this entire process he died after four days of fever.so his generals divided his empire into Parts of which included the Seleucid Empire that reigned over the eastern Mediterranean to parts of Afghanistan until a Parthian (eastern Iranian) confederation swept the Seleucid Empire aside and kind of De-Hellenized Its controlled lands.but despite all that The Parthians adopted many Greek customs and even started a very Helleno-Persian like civilization.

[Satapatiš]

Rare, let's see...
Ionian uprising 499-493 B.C. Though only because Persian rule in Ionia was relatively fresh and they had awful understanding of Greek internal politics, later they did better with Greeks under empire. Still, after Greco-Persian war Ionia easily switched to the Greek side.
Nonetheless, not really reliable region in this time.
Egypt. Revolted circa 486 B.C. then again circa 465 B.C. And again in 404 B.C. Though probably because Egyptians always were xenophobic. Or rather because of local nomarchs. How fitting that after "liberation" by Macedonians Egyptians came under the rule of people who had no respect for their culture. Unlike the Persians.
Again, it's hardly a stable province. Ever. And too close to the Syria and other key regions of the empire.
Mesopotamia. First Gaumata's revolt in 522 B.C. followed by Nidintu-Bêl/Nebuchadnezzar III (whatever name he liked more) and immediately Arakha/Nebuchadnezzar IV. And then again in 482. B.C. At any rate it wasn't a stable province these times.
Though later Achaemenids had much better situation and the only real internal threats were satraps and other family members, that wasn't the case under Darius and Xerxes.

Which reminds me - since RoP is on BI.exe, we are definitively in need for emerging rebel factions for conquered Ionia, Egypt and Babilonia. And a loyalty factor for generals. This isn't the Persian empire without backstabbing satraps.

[Blatta Optima Maxima]

'Gaumata's revolt' wasn't civil rebellion, it was a succession dispute. And I am inclined to think he might as well have been the legitimate Bardiya, with Darius being the rebel. Since Darius commanded the king's bodyguard, one could say this was the typical 'Praetorian coup', characteristic for Imperial Rome.

[Satapatiš]

He was Bardiya, unless someone is really inclined to believe not only what Darius said, but also that Bardiya's wife was apparently blind, stupid and autistic.
Though he had supporters in Babylonia since Persian kings were also crowned as kings of Babylon.
There is also something fishy with Kambyzes death.
Still, there was enough unrest in the empire under Darius and his son that no sane monarch would go on conquest without leaving in home enough loyal forces for keeping things quiet. And Xerxses actually had this kind of problems with Babylonia at the end of his invasion of Greece.

[Blatta Optima Maxima]

Indeed, Cambyses died a rather suspicious death. My take on the whole thing would be that Darius, acting in concert with disaffectioned nobles, murdered Cambyses and then marched on Babylon to defeat the legitimate heir. But while the two princes were at each other's throats, the provincial feudal magnates and native leaders saw an opportunity to break away and carve out kingdoms of their own, although their efforts were negated by Darius, now king, crushing them all.

[Satrio Eko]

Hey,what are you discussing here?
Civil rebellion or Achaemenid armies?
Back to topic,while persian respect different culture of their subject.why they are still revolting
and Alexander the (Bastard) doesn't respect the natives culture and form a major helenizzation.but the citizen under the diadochi are content at the reign of person who doesn't have respect of their culture?!

[Satapatiš]

Actually, it is related - Achaemenid armies and civil uprisings.
Because it is pointless speaking about supposed vast hordes of Persian troops in Greece, when, regardless of reason for rebellions, you have to left considerable part of your forces for peacekeeping your empire.
Regardless how big Persian army was in reality, no more than half of it (at best) could be used for external invasion.

As for rebellions, they never had anything to do with any perceived Persian oppression in the conquered lands.
Egyptian uprisings were caused by local nomarchs. Prior to the Persian conquest Egypt wasn't exactly united. There was always struggle for power between king and local noble rulers. Like in any other monarchy, including Persian. Now, after conquest Persian kings were crowned as Egyptian pharaohs and they did continued this struggle, wanting strong authority over temples and nomarchs - and unlike late weak Egyptian kings, Persians had power for enforcing it. Of course local nobles weren't particularly happy because of this.
There was also Egyptian mentality. Egyptians always perceived themselves as a better and chosen people, they were extremely xenophobic toward others, to the point where Egypt was rejecting most forms of foreign progress and falling into stagnation.
Another reason is that for Egyptians pharaoh should remain in Egypt, due to religious and magical reasons.
Of course Persian king couldn't do it.

Greek initial uprisings were caused probably because of Persian lack of knowledge about Greek internal affairs. Persians were supporting tyrants in times when tyrant rule was no longer popular. Though after Greco-Persian wars Achaemenids corrected this approach and they had no more problems with Greeks.

As for Babylonia, its is again case of local nobility. They either wanted greater participation in the empire, or ruling Babylonia on their own. An average Babylonian probably was giving no damn to this.

As for Hellenic monarchs - rule was enforced by military strength and Greek military colonisation. Besides, Ptolemies also had to fight with Egyptain rebels and for a long time part of Ptolemaic Egypt around city of Thebes was nothing else, but independent rebel territory.
Hellenism very often was oppresive and destructive for local cultures. If you weren't Greek or rejected hellenisation, you were citizen of second class. Again, Ptolemaic Egypt is a good example.

Local citizens under diadochi weren't always happy, they simply had no choice.
Persians never had this kind of attitude, too bad for them. In fact they were pretty lenient in dealing with rebellions. If these would be under Assirian or Roman rule... You would be really lucky finding Babylon on the map or living Greek in Ionia.

[Kaveh]

Very true The Ionion revolt was indeed caused by a great misunderstanding of Greek internal affairs.
But It is also undoubtedly fair to presume that the magnitude of the unnecessary Bloodshed caused by the Greeks was completely unjustified.
and also fair to say It triggered an Acheamenid invasion due to the previous intervention of Greek city states fearful of the Persian expansions in Asia minor.

[Blatta Optima Maxima]

Satapatiš - numbers of men on campaign are limited by logistics far more than by politics.

And besides, most conquered peoples were forbidden to arm themselves as an official imperial policy, and thus it is unlikely that they all showed up to fight at Greece for that reason alone. The three most important sources of manpower were the feudal aristocracy who fought as cavalry, the Persian freemen and mercenaries from within and outside the empire. Other sources were levies from the few still militarized regions of the empire and the personal retinues of kings, satraps and great nobility.

Over time, it was the role of cavalry retinues and mercenary infantry (Aethiopians, Greeks, Chalbyians, Cadusians, Hyrcanians, Saka and even Indians) that increased.