It seems like a pretty bold move to make a DS racing game that doesn't use the stylus to control an onscreen steering wheel, but then it's a pretty bold move to make a DS version of a PC racing game. According to Trackmania DS producer Dan Fletcher, the team felt that touch controls "do not fit" for driving.
That's not to say that the stylus has no place in Trackmania. The one aspect of the game that is perfect for touch control -- the editor -- uses it awesomely. In addition to the basic stylus control used to place tracks, Trackmania DS also features a "free draw" mode that lets players draw tracks directly!