Steel is preferred over aluminum because it wins in two key categories: elongation (how much the material will flex before it plasticises or fails) and endurance limit, or fatigue life. What this means is when steel and especially titanium are flexed, they retain their strength. They can be flexed again and again and never weaken until the material hits its endurance limit...for steel that's a pretty long time, Ti it's virtually forever. Aluminum loses strength any time it's flexed, and since its elongation numbers are low already, the chance of catastrophic failure looms. One of the myths about aluminum is that it's a very stiff material. It's not...the elastic modulus is actually much lower than steel or titanium. But since aluminum is about 1/3 as dense as steel, the tubing can be made very large to compensate. And since flex is aluminum's Achilles heel, most framebuilders will overcompensate, shooting for a finished product roughly 3 times as strong as steel (sometimes more, sometimes less depending on application).

Now (just like carbon, IMO), it's *possible* to design a strong, light and most importantly, *reliable* aluminum frame with the proper application of fistfuls and fistfuls of cash...cutting-edge alloys, expensive and time-consuming manufacturing processes and design tricks like hydroforming and internal or external butting. Would the market support such experimentation? Probably not, since there's no demand for anything to replace steel.