Precisely, they are designed for racing, and racers want every performance edge they can get which means as little rotating mass as possible. They are made to run on a smooth flat surface, with no obstructions. One of my friends had one of his MT fronts go down driving back from a run...it happened in the pits (thank goodness), ran over a hard piece of plastic (which is why I mentioned "plastic" a few posts back) and the tire went down INSTANTLY. He ended up having to pull out of comp because he didn't have a spare.
If you think about materials of construction for each tire, one could argue that the "street tire" is much safer due to it being less likely to have an instant flat situation like mentioned above.
