I guess the general "hype" about the Marian troops has several reasons, some ingame, some not.
Ingame reasons include the already mentioned much wider range of recruitment and the easier troop management. Reasons that are not related to the game are for example the focus on Rome and especially (post) Marian troops in movies and books (to prove this point: name the Hollywood movies that are about pre-marian times and therefore feature Camillan or Polybian soldiers, and then in comparison do the same for later stages of Roman history
).
If one says "Antiquity" the chance is almost 100% that the one hearing it instantly thinks "ah, Rome." or "ah, Caesar" or "ah, Augustus".
In school and highschool and university there is a strong focus on imperial Rome, too.
To me, personally, the roman history is most interesting up until 146 BC (when they destroyed both Korinth and Carthage), to be honest. After that comes a long period of "yawn" (huge exaggeration!) until finally the Empire begins to fall apart and to struggle (since 3rd/4th century AD).
On the other hand, playing a Rome campaign in EB (talking about EB I here, haven't played prolonged SPQR campaigns in EB II yet, waiting for the Polybian roster before I do so) always had this very motivating "you'll be eventually able to field a really professional and
professional-looking army" thought. That thought kept me playing them for hundreds of turns.
Although I have to admit that my anticipation had more to do with the Imperial army than the Marian one. The Imperial Archers were one of the most beautiful units in EB I and over the years I became quite obsessed with reaching the point where I could field them. That I only managed to get the Imperial reform once in about 15+ campaigns with them, only added to the thrill.
(On the other hand I liked the marian line troops much more than the Cohors Imperatoria so that in my ideal game we would have Marian troops + Imperial archers and no Imperial reform at all
).