I think it was part them willing to take risks. But also not talking down to their audience. Don Bluth talked about adding dark and scary scenes to his films so that the happy endings feel more satisfying.
With recent animated films for kids, especially from Disney, they are trying so hard to not be scary so that they can also appeal to the toddler demographic.
Visually, it was around the time when they were using computers to aide with animation. And I think that sort of helped them animate more complex characters, vehicles, and backgrounds. I feel like a lot of the animated films from the 90s and early 2000s look very dynamic because of it.
I always think of the scenes from Tarzan