It's probably a selection effect you see among internet users.
People who frequently post on forums are overwhelmingly INTP/INTJ. Those are the same personality types that tend to get attracted to theoretical+impractical pursuits such as astrophysics (and the more impractical parts of theoretical physics), as well as fields that seem to have a "soul" (as described in the URL below).
I've been to several other science forums (as well as Quora), and almost all of them are heavily overrepresented by physicisists (and particularly, astrophysicists at that) - neuroscientists and evolutionary biologists often make up at least half the rest. The blogosphere is also heavily overrepresented by astrophysics, evolutionary biology, and neuroscience (which are pretty much the top three "sexy' science fields on the Internet). In other words, science with soul sells:
http://blogs.discovermagazine.com/gnxp/2010/08/science-with-soul-sells/
Just look at
http://www.quora.com/Quora-Users-by-Interests-Segments/Which-active-users-on-Quora-are-scientists, for example. Practically *everyone* is in one of the three fields with a "soul": physics, neuroscience, or something related to evolutionary biology. Aside from those three fields, you see practically no one from any other field. We're also seeing this disproportionate representation on the Reddit AskScience community.
With that all being said, it's not that popular of a field among undergraduate majors, and there is still *plenty* of demand for undergraduate research (astronomy is pretty much the *easiest* department to get non-menial undergrad research in, apart from the earth, atmospheric, and planetary sciences).