From my limited experience hunting from a job in Austin, I think one of the reasons is that there literally aren't that many firms (small or large) doing software engineering.

The other vibe that I get is that culturally most of the older companies are from a different era which kind of doesn't make them too attractive a destination for a new graduate. Surprisingly enough the situation for EE grads is much more rosier: Anecdotal evidence over the past three years suggests that most of them get hired and stay in Austin.