I'll attempt a straight answer: Aluminum has a "memory", similar to glass. Every little jolt, slowly weakens the frame. Steel and Titanium do not have this "memory". After enough little stresses on anything aluminum, it becomes brittle and easier to crack. I used to hear that the lifespan of an aluminum frame is about 5 years. That makes sense to me. Steel is forever, as long as you can keep rust away. Aluminum tends to also be a harsher / stiffer ride, but I'm sure you already know that.