Most hand drawn animation wont't actually vectorize well, especially the ones from the hand drawn era, where every cel is a real painting:

https://caseantiques.com/item/lot-738-the-simpsons-animation...

Look at all that line detail. It creates subtle gradients in the final film, which don't readily transfer to vector graphics.

In this case, I think with some better pixel-based filtering before vectorization, they could've gotten a better result.

However, the much bigger problem is that the vast majority of animation uses paintings for backgrounds that aren't just solid lines and colors.