That and people didn't know how to push the envelope. You see things even today on various sites about how the NES's palette was so limited and crappy, but the programmers could alter the palette themselves if they set the right flags. But yeah, the NES's hardware kinda limited this, at least until the newer chips were integrated.
This isn't too far off the old DOS and Win95 games, like King's Quest, just a little more spruced up.