Myself, I suspect the most plausible explanation is that interstellar colonization is simply a lot harder than we think. Nobody yet has managed to build a self-replicating probe that will actually survive through interstellar space intact and manage to set up a colony, and it's not a given that anyone ever will. Add to this the fact that even if it were possible, it could be horribly expensive, with a return on investment at least hundreds of years away.
Kaj, I'm surprised! I think you should update against this explanation; of all the technologies discussed here (superintelligence, Dyson spheres, megascale engineering, nanotech), space colonization is the only one that is so simple that even contemporary humans have a definite recipe for doing it that would probably work. If humans can do it, to a superintelligence it would almost certainly be trivial.
An uplifting message as we enter the new year, quoted from Edge.org:
A few thoughts: when considering the heavy skepticism that the singularity hypothesis receives, it is important to remember that there is a much weaker hypothesis, highlighted here by Tegmark, that still has extremely counter-intuitive implications about our place in spacetime; one might call it the bottleneck hypothesis - the hypothesis that 21st century humanity occupies a pivotal place in the evolution of the universe, simply because we may well be a part of the small space/time window during which it is decided whether earth-originating life will colonize the universe or not.
The bottleneck hypothesis is weaker than the singularity hypothesis - we can be at the bottleneck even if smarter-than-human AI is impossible or extremely impractical, but if smarter-than-human AI is possible and reasonably practical, then we are surely at the bottleneck of the universe. The bottleneck hypothesis is based upon less controversial science than the singularity hypothesis, and is robust to different assumptions about what is feasible in an engineering sense (AI/no AI, ems/no ems, nuclear rockets/generation ships/cryonics advances, etc) so might be accepted by a larger number of people.
Related is Hanson's "Dream Time" idea.