Another theory: The so-called Flash Sideways Timeline is a place the Losties go when they die. As Christian Shephard said, "this is a place you all created so you could be together again." When Jack & co. detonated the bomb in the hatch, it did two things. (1) it returned all the Losties to one time line on the island so they were no longer split between 1977 and present day. (2) Because "whatever happened happened" is an enduring concept, the Losties could not change anything in detonation of the bomb in terms of experiences in their linear timeline, as they had erroneously believed. Instead, it created the flash sideways, the alternate reality they could experience only when they died.
Only the Losties are there because as candidates, they were special. While Jack died in turning the source back on, they all in some way gave their lives to save the island and therefore humanity. Their sacrifice allowed them to have another opportunity to resolve their life issues prior to moving on or a chance to see what might have been.