Because, shock of all shocks, it's possible to create similar and entertaining experiences. I complain a lot about the modern Sonic games because I just don't find them fun. Obviously we can't go back to the days of the classics, and why would we want to? I want something new that doesn't suck.
Like the aforementioned Smash Brothers Melee. SSBM is much better than SSB. It improves on the original concepts and adds a ton of new ideas. However, it doesn't completely break what made the game fun, like the modern Sonic games do. That's my objection. I have no problem with 3D and improvements. I have a problem with the fact that the new games just suck, and they need to remember what made the original games fun before they can improve on the concept. You wouldn't build a school on the foundation for a townhouse. SEGA should build on the original concepts in the classics, instead of trashing the physics engine and creating something totally new, which I just don't enjoy.