>>151069532
Because it was simpler in the eyes of many. Eggman causes trouble, Sonic rushes in on the scene, Sonic beats up Eggman and his minions, repeat for most of the Classic part of the series.
And for Mario doing this? That's just fine, but at the same time, Sega wanted to push Sonic into giving players a story to care bout, whether or not the story was good jumps from game to game, that's great too.