Actually, now I'm left with another question.. why the heck to people keep praising Sonic Rush? I personally couldn't stand the game. I think someone else in here already pegged the main reason, which was the horrendous level design that sent you flying off into death at a moment's notice or the confusing level design itself that left you wondering where the hell you were for half of the stage. The Sonic Advance games were like that.
The Sonic Adventure 1 and 2 games.. were not like that, which is quite possibly one of the reasons I enjoyed them on the Dreamcast so much. Sonic Heroes.. eh.. Shadow.. good music, interesting idea, but it just felt like they were running Shadow's character through a meat grinder and wrecking his character more than they did Knuckles' by making him listen to Rap constantly (which was stupid).
I still think that it's possible to make a good Sonic game, I'm just not sure where the true fault lies. The people making the games for making them poorly, the fans for buying them regardless, or the company for not LISTENING to what people are saying are making the games unfun.