Right now, we still don't know why everyone didn't flashback, या stay in present time. When Sun, Jack, Ben, and Desmond went to see Eloise Hawking in "316" she told them that if all of the Oceanic Six didn't go back the results would be "unpredictable". I think that, because Aaron didn't go back with them, they were विभाजित करें, विभक्त करें up between the two times. But really, we don't know why the people who travelled back did, या why the others stayed. Hopefully we find out soon!
I'm guessing the island didn't need them at that time. It took who it needed via white light before the plane went down (most likely because they play some major role in whatever is about to go down) and the ones it didn't need in the past, it left them in the present. Its just another step on the "whatever happened, happened" latter.