I think the difference between the romans and the hellenistic nations lay in the proximity to a people with the expertise and the iron deposits required to equip a multitude of men with chainmail.

The romans had nearly always had a relative proximity to the celtic cultural sphere, especially after they had conquered Gallia cisalpina.
There they had access to ferrum noricum and a mainly celtic population to manufacture them in great numbers.

Whereas the Seleucids had the Galatians, who, as you pointed out earlier, were either poor raiders or mercenaries, so their capabilites to do the same IMO is questionable.

What are your thoughts? Do you know how many iron deposits were in Galatia?