Maintainence. The bikes with exposed chain drive need to be oiled every so often where as a belt, if it wears out then you just need to replace it, plus rust. Even though stainless is rust resistent its not rust proof because of the small amounts of iron, now there are certain types of stainless that do not have iron mixed in the alloy but they lack strength in comparison and are less rigid (more flexable). Now chain in a car is self oiled and is pretty much sealed, for the most part, from moisture.