They want people to remember it but not be able to relive it. Hence why we haven't seen any more Sonic racing on foot since R. Seriously though why does he need a car or a board/bike, For the supposed fastest thing alive getting outrun by somebody is pretty pathetic.
Number one, if they did do a racing game with him on foot, it would have to be at least as fast as the current games. Unleashed and Colors seem to be pushing it a bit in terms of ANY kind of racing. Second, the last attempt at car racing, Sonic & Sega All-Star Racing was my own personal little GOTY, being racing heaven for me, a fun game which lasted me for a big while, had a ton of favorites, and was way, WAAAAY better than Mario Kart.
And Sonic R provided me with lots of great, fun moments back then. No, it doesn't hold up nowadays, but it's still a fun game to play.
...and Sonic X was unfortunately a huge dissapointment. TERRIFIC art style, I was impressed by it, but the plot was horrible and it was just your general kiddy Pokémon/Digimon style of anime, where a little boy of spiky hair is the Mary-Sue around the place and learns how to be strong and brave to act as a role model for kids watching. It's really too bad. The first episode showed such potencial.