Part of it was the video mode. EGA 640x350x16 had 16 simultanous colors, from a palette of 64 possibilities. And non square pixels as a bonus.

They might have made better choices from the palette, but the limitations were severe.

If you really want to stab your eyes out, CGA had a mode with white, bright pink, light blue and black. I remember playing Keen on it. I've never seen that mode used for anything nice.