Carbon has become the material for competition bikes but steel and titanium still have their place as they are considered "classic" and "sexy". Why is it that aluminium failed to remain an attractive material for frame building?
A large part of of that is because of marketing and economics.
Aluminum has become so cheap and commonplace in our everyday lives. The ease at which aluminum can be manufactured and hydroformed into different tube shapes has driven its cost down. You can buy department store bikes for $30. Aluminum technology has "matured", there is very little that can be done with improving it.
People relate "cheap" to "inferior".
Carbon fiber is a "new" technology, even if CF has been around since the 1960s. Manufacturing carbon fiber frames is a labor-intensive process (and carbon lay-up critical), which tends to drive the cost of manufacture up.
Titanium is hard to work with, literally. This generally limits tube shapes to straight ones. Titanium manufacturing is a specialty and this drives its cost up. Its availability to a small market makes it more desirable.
Steel is well, steel. Like aluminum steel technology is already matured. Since it has fallen from the mainstream choice of frame material it has acquired a "classic" status. There's always a market for classic things - the oldies and the fans.
A lot of people buy bikes knowing very little
(I'm putting myself on the wire here) of the technical merits of the frame material. Most of them just buy into the marketing put forward by the companies, driven by the market and social economics.
Is it just me, or do more forum members wonder why aluminium seems to have become a niche for high(er) end bikes?
Companies like Cannondale has heavily invested in developing aluminum (the CAAD). Giant has always made their high-end TCR line to include aluminum. Other companies like Specialized have "resurrected" aluminum as well.
When people hear aluminum they think "common". But these companies are doing something new and different with aluminum. And being "different" sells.