It is my understanding, and I may be wrong, that steel is much more resilient. Each bump, drop, rough road, etc. causes the frame to flex. Steel can supposedly be subjected to an infinite amount of minor flexes while other materials will eventually fail under the same conditions. Does this matter to the average person when it comes to frame materials? Probably not. I own three bikes. All three are aluminum frames. Forks on these three bikes are carbon, suspension, & steel. Supposedly, the repeated flexing failure concern requires aluminum forks to be super stiff, which leads to a harsh ride.