It's a great point: bang-for-the-buck, durability, usability, it's hard to beat Al. I have two, an old 26" mtb, and a 2000 Bianchi road bike. They're easy to live with, and ideal for the vast majority of cyclists.
There is the obvious difference in ride quality, to those of use that notice these things. But most do not notice and don't care, they just want the bike to work, and Al works really, really well.
I think it's major problem (if it has one at all) is one of perception: it's common. Nobody - especially here on a bike-specific web forum - likes "common". Steel, "vintage", Ti, are not "common". Carbon may have already reached a point where there's "high-end" carbon and "inexpensive" carbon; and as it gets more common, it will suffer the same perception as Al.
But I'm an Al fan, I think it's a great material and I'm a big fan of "inexpensive" yet super-functional.
Should be some interesting replies in this thread