I think materials which are light are able to be light because they have high strength. and strength and hardness of a metal are directly proportional (cf ductility, which is inversely proportional to strength). So a lighter material like carbon fiber, because it is stronger, will be stiffer, more brittle, less ductile. And that means less flex, which means that any bump in the road will be more directly transmitted to the rider, as opposed to steel which will "flex". I'm not a bike expert but I know a little about metals.