Just as there was back when we were trying to figure out what made certain survivors "special" in the minds of the Others, there's a lot of discussion now about why Jack, Kate, Hurley and Sayid were cast into 1977 while Sun, Frank, Ben, and Locke ended up in 2007. Surely, the discussion goes, there must be a good reason - having to do with the recreation of the original crash, with their personalities or actual physical bodies, or perhaps (and this one is my favorite, since it implies some new mysteries) because they're already present on the island in 1977.
What I suspect, though, is that the most obvious answer will be the right one - that it's simply that The Island wanted some of them in 1977 and others in 2007 in order to complete some sort of job. This is better than revealing that it's totally random, but only barely. I'm reminded, again, of the first season of Lost, when people were trying to figure out what the common bond among the characters was - that they were all in need of redemption, that they had all killed someone, that they had all cheated death, etc. - so as to explain why they ended up on that island. But as the show goes on, it seems increasingly likely that the only reason we'll ever get is 'because they were supposed to end up on the island', which is really just another way of saying 'because The Island wanted these specific people'. And that's just disappointing.