>>12016217 (OP)
Simple answer : Nintendo wasn't experienced with the higher fidelity (when comparing to 8 bits consoles) pixel art yet.
And not just them.
A lot of Super Nintendo games underutilized its core capabilities.
It could output more colors per screen than the Mega Drive, and yet, many times the colors looked worse.
Why?
Pillow shading.
There was too many colors to choose, devs didn't know how to use them properly, with appropriate, organized value tones and a well defined light source which often lead to very bland use of colors.
See how Capcom reduced the amount of colors in SF Alpha series, yet the sense of form/volume is as good if not better than SF2.
https://phfranqueira.medium.com/como-fazer-pixelart-pelo-criador-de-tropicalia-19d45b429095
So, in short, Nintendo didn't know how to use the SNES capabilities.
Right now there are hacks such as Plumber for All Seasons and the upcoming Mario Stellar Destiny that make SMW look much better (although different in art style indeed).