Bear with me, I don't know ANYTHING about superman. I just watched Man of Steel out of curiosity. I have the feeling that this may have been addressed in the movie, but I can't remember.
Man of Steel spoilers
Krypton is collapsing because they harvested its core. When Jor-El speaks with the Council guys, they seemed to be oblivious of what to do next. Jor-El then says something along the lines of
Do what our ancestors did, look at the stars!
That seemed odd to me. Jor-El's suggestion is pretty obvious, unless Kryptonians didn't own other planets or major colonies at all.
Additionally, when Kal-El's ship triggers a distress signal, Zod seems to be the only one to respond, further suggesting that there really are no other major Kryptonian communities out there.
Didn't Kryptonians own other planets/cities/colonies other than Krypton?
I remember they did send lots of exploration ships before. But I'm not sure what happened to them (other than the one buried in Earth).