Because Sega does what Nintendon't.
In 1990, we sold most everything and moved to Philadelphia, my NES and 4 games got sold at a flea market. Once we were set back up by 1991, I saw Super Mario World, and thought it looked great.
But then I saw Sonic, I knew that's where I wanted to be, so I got a Genesis.
The Genesis just hit the right vibe at the time, but I think by 1994, they about evened off when Nintendo stopped being such a nerd.
Sega tapped into the hip hop, grunge, and other cool culture of the time. You had near arcade perfect (at least for the time) conversions. Unique quirky games like Sonic, Earthworm Jim, Toejam & Earl, The Ooze, etc. Blue Sky Productions was on fire at the time, look at Jurassic Park for the Genesis vs. whoever made the JP game for SNES.
A lot of quality coding going on for the Genesis, whereas the SNES had some really sloppy work from 3rd parties.
I think the fact that the 68000 runs the Genesis is part of why the quality of code in Genesis games was great, I hear all the time of a lot of people from the ST or Amiga scene broke into Genesis games when things drifted that way. So they knew tricks they figured out on the Amiga and ST and they could push the Genesis in ways they didn't know how on the SNES.