Fun fact: TMNT has had acting from over 55 VA's since it began--and this is taking into account actors with more than one established name. Chaotic also has a crapload of VA's named in its credits.
The problem here is two-fold:4KiRAB' overuse of their top tier (Schemmel, Grayson, Green, Stuart, Griffith) for most of their shows, at the expense of some of the up-and-comers.The fact that it's incredibly hard to tie a VA with a non-established voice to a particular character, thanks to 4KiRAB' practice of not crediting actors for specific roles. I mean, look at the cast list for TMNT's fourth and fifth seasons:
And that's without counting the regulars. Sure, you have a good amount of established talent in there, but who can tell me, without research, what character David Moo voiced? David Chen? David Zen Mansley? (ETA: No, I did not choose those three for their David-osity--I didn't realize it until I made the post.)
Also, thought excercise: name VA's who you think should be getting a lot more work, based on their abilities.