Because the experiments turn out bad?
In addition, very few of the experiments add to the core experience. The ones that do become mainstays (boost meter, red rings, wisps apparently). The other ones, the swordplay, the werehog, that sort of stuff, go against the core experience. Sonic is about going fast and exploring levels to go even faster to the point where you can casually breeze through a level at high speeds. Werehog is about playing kid-friendly God of War. God of War already does God of War, and it does it pretty good. It's not what people want in a Sonic game, it's not the reason they bought the game...
And then there's the reworks, the ones that try to change the whole game. Your Sonic '06, your Sonic Boom, that sort of stuff. When those experiments happen, what once was is now lost, and the result is absolute garbage. It's like the lost the guide on how to make Sonic games and had to start over from scratch, and when they do finally make a new guide on how to make Sonic games and start making some better products, they go off and lose the guide again and drop another rework...
And that's why people don't like the experiments. They aren't what the people want, they play bad or other games do it a lot better, and sometimes they actually ruin everything. Experiment if you must, but make sure that they play good and don't feel obliged to drag your most iconic character to the firing line each time you do it...