Another possible explanation is that interstellar travel is impossible. Perhaps there's too much debris between star systems, and it's not possible to create materials strong enough to make a spaceship that can withstand a collision with the debris at the speeds required.

This doesn't quite explain the lack of communication signals. But if nobody can colonize the galaxy, then there would be less sources of signals.

BTW, the Wikipedia article on the Fermi Paradox (http://en.wikipedia.org/wiki/Fermi_paradox) does a very good job of detailing all the possible explanations.