I didn't say there werent any - just that its not playing to the strength of the platform. You could in theory drive to work in a Formula 1 racecar, but its doesnt excel at a round-town car - but put it on a racetrack...
Point taken. I'd still like to see some long rpgs on the handheld. After all, Pokemon was a big success so length isn't necessarily the problem (some may cringe at it, but leveling up monsters is rather rpg like to me...), but some of us would feel a bit old to be collecting Pikachus (no, not bashing, I'm just saying some fans have grown out of it, like myself). But I wouldn't mind collecting experience to be better with guns instead...
As for why, I'd say a long rpg would be nice because there aren't ishloads of them on the system, for one, and because, as much fun as Mario Kart is, I can't stand to sit and play it for more than an hour at a time. If I was visiting a relative, for example, and can't access a home gaming console, it would be nice to have an immersive game you could carry around, throw time into in short bursts if necessary but can easily throw hours into (think Age of Mythologies for the DS, or Chrno Trigger). But that's just my (clearly biased) take on why Fallout on the DS would be awesome.