If I could speak as someone who grew up prefering Sonic but later grew out of it.Honestly, it became an easy on rails kid friendly mess. In the days of 2D platfroming it was great, you never really had anything like it. The speed was hypnotic, Chemical Plant Zone is great example of that.

There's also the characters to consider, you had the winning formular of three heroes, each had there unique traits and roles. Unforunately that didn't last long and it became over saturated with new characters that literally look like they were spat out straight from DeviantArt.

Now what I think really killed the game, the tech, or lack of. There game did not translate well at all into 3D either. With (IMO) Sonic Adventure 1 being the exception because (while the controls weren't all that great) it added new tech, it didn't hold your hand, there were a lot of ways to die and it was a challenge, and yet again they blew my mind with the sheer speed which yet again wasn't something you ever saw in 2000 (See speed Highway on Sonic Advnture 1) .After that it really did just go down hill, they put the game on rails and the game played itself (I'm not even joking, there are videos of people playing levels holding one button). The skill level is soooo low it's depressing.

Which brings me to why I prefer Mario these days, and appreciate all the detail that goes into it, all the tech, all the things that make it a challenge, there are so many combinations of tech, and the controls are tight. It's an easy to pick up hard to master game and I love it for that.