One major reason is sprites: Amiga can display 8 4-color sprites, or 4 16-color sprites, and the colors are shared with the bitplanes.
SNES can display 128 16-color sprites, and the sprites get 8 palettes all to themselves.
This leads to much more colorful-looking visuals on SNES. Since Amiga is all bitplanes, enabling more colors and higher resolution results in a massive performance hit. Most game entities would need to be blit on top of the background, and then the background "restored" every frame that entity moves. SNES' native support for multiple tile layers and good sprites means that the CPU can do a lot less work to achieve a lot more.
Amiga can do some very cool stuff that SNES can't, especially with the blitter, but SNES is much more practical and powerful for video games.