1) TMNT just fucking erupted onto the scene and made millions and was an utter monster of a success story made by two guys who just gave away their original parody comic as a convention door prize. TMNT never really left, either; the logest break the franchise had was from 97 to 2003 and we're still getting TMNT stuff with barely a break between reboots. There were countless imitators who rose up and subsequently fell; I think Battletoads is the only survivor among them.
2) As others have mentioned, there was apparently a tax break on non human character toys at the time, which mean toymakers had it made back then as long as the trend of animal cartoon/toy combo kept going.
3) As a general thing, even nowadays kids media is often presented with animal or anthro characters. Talk animals get the kids attention Paw Patrol has been a thing for little kids for, what, five years or something now? And Scooby doo seems to be fucking eternal.
4) While a lot of this shit was going on, Nickelodeon/Nicktoon style humor became more and more popular; sure Ren and Stimpy and Rocko were anthro/cartoon animal vehicles, but the Nicktoon influence shifted things alot into the 2000s.>>107229816>>107237134
...They can't just make female aliens?
I mean shit, Voltron fucked up some shit at the end of their run, but even they got that right.