A lot of traditionalists will keep making excuses - but the fact of the matter is this...

professionals care about performance. One of the variables that they look into is stiffness, the other is weight savings. Comfort comes later.

Steel doesn't offer an advantage in weight to stiffness performance - it can be made stiffer than composites, but it'll be heavier. It can be made light, but it wouldn't be as stiff. Composites can also be more refined in design - you can't really engineer steel to be more flexly in the vertical axis while being stiff in the horizontal. With composites - the designers can be more selective about that sorta thing...

since the balance between stiffness/comfort, stiffness/weight, reliability/weight is usually hard enough to define and varies with individual's butts - composites make the design process easier because you can control just a bit more of the construction... instead of a relatively homogenious material that you can only affect the diameter, wall thickness and other structural dimensions...