Lots of possible reasons...
*don't want a big investment on a first bike.
*steel is just preferred by people that commute
*bike was already sitting around
*older bikes are more likely to pop up on craigslist
*established standards have more readily available parts
*mass produced bikes have staying power

I personally have no pressing reasons to run out and buy some new fancy carbon race bike. I'd love to have one, just because N+1, but on that same note I could probably have three nice used bikes for the same price. I do try to stay in the last fifteen or so years with 130mm hub spacing, threadless stem, 1-1/8 steerer.

In my riding group, there are just as many modern steel commuter bikes as their are retro/classic types (in which case, mostly fixed or single-speed). Steel is preferred among my local peers. I can't, nor do I have a desire to keep up with the guys that are buying $3k carbon bikes, earning KOM's, and riding in 25+mph pacelines as they blaze through stopsigns and lights.