Why alu?
-Lighter than steel. I'm not gonna ride a tank, and in case you'd say there are lightish steel frames: I'm not gonna ride a bike that dents when I lean it against a pole.
-Cheap and easy to find
-Doesn't rust
-Did I say it's light and cheap? And then it doesn't rust. What else do you want?
The superior ride of steel is probably all fictional.

In all fairness, I'd much prefer Ti, but I don't have the cash for it (yet). As to CF, it wouldn't live long as an urban fix... It's a tossup between theft and getting destroyed in a crash... on or the other would dispose of it within a year or three.