We don't. I used to prefer the modern look.
However, I feel like modern Sonic's speaking voice doesn't fit, and never has. Except in Shadow and Unleashed. The fact that he doesn't talk as Classic Sonic was a godsend for a lot of Sonic fans because his voice is so grating, coupled with the incredibly campy writers for the games' dialogue.
Also, Classic Sonic is just a better design. He's still got an 'attitude' but he's also cute and likeable on the levels of other mascots like Mario.
And lastly, most of modern Sonic's games actually suck. This is coming from someone who grew up with several of them.
And love them.
EDIT: But I would prefer, if they redesign Sonic again, that's it's a hybrid; the colors (and eye color) and shoes of modern Sonic, with the stoutness and shorter spines of Classic Sonic.