Because according to Star Trek quantum reality theory all possibilities exist. Data mentionned that theory in a TNG episode : ''Quantum reality, or quantum universe, was the term used to describe the alternate timelines in which all possible outcomes for any event take place. Each quantum reality was its own separate parallel universe characterized by a unique quantum signature which could not be be altered. All matter resonated on a quantum level with this signature within the universes. Before 2370, the existence of different quantum realities was considered only a theory in quantum physics. Under normal circumstances quantum universes remain separated from one another by barriers. (TNG: "Parallels")''
Link : http://en.memory-alpha.org/wiki/Quantum_reality
Data's explanation of quantum reality:
''"Many worlds" in Star Trek
Some serious problems with Star Trek time travel can be solved once you accept the "many worlds" theory:
"City on the Edge of Forever": When Doctor McCoy jumped through the time portal, the other crewmembers on the planet's surface perceived the sudden disappearance of the entire Federation. Supposedly, he changed the past so that the Federation was never created. But that is impossible because the other crewmen still existed. They still had memories of the Federation. They still had Federation uniforms and Federation weapons. The "many worlds" theory neatly explains this problem: McCoy and all of the people on the planet's surface were all transported into a timeline (or parallel universe, whichever you prefer) in which the Federation never existed. The original timeline is not destroyed, thus explaining why they still remember its history, but they can no longer perceive it or return to it. When Kirk and Spock jumped back to "fix the damage", they caused everyone to jump into another timeline, in which the Federation was founded again, but with slightly different events surrounding Edith Keeler's death. This is not the same as "going home", but as far as they're concerned, it's good enough.
"Star Trek First Contact": When the Borg jumped into the past, the crew of the Enterprise perceived the disappearance of the Federation's entire history. This is impossible because they still exist, and they still retain all of their memories, equipment, history files, etc. Data suggests that they were somehow "shielded from the changes in the timeline", but he doesn't even attempt an explanation of how this is possible. The "many worlds" theory provided a neater explanation: they were dragged into a new timeline by the Borg sphere's "temporal wake", and when they stayed in the wake long enough to perform a similar jump, they ended up in yet another timeline. In this new timeline, they tried to "fix" events so that they unfolded more or less as they remembered (albeit with an orbital bombardment of Cochrane's launch facility which didn't occur in their original history). Note that the "many worlds" theory also explains the biggest conundrum of STFC: why the Borg fought their way to Earth before performing the time-jump, instead of making the jump from the safety of their own territory. The answer is that a time-jump would move the travellers to a divergent timeline but it would have no effect on the original timeline. Therefore, it would do the Collective no good. You might ask why they performed the jump at all if this is the case, but the Queen's attack had failed and she was facing imminent destruction. A jump into a divergent timeline would not change history in her original timeline, but she may have found the prospect preferable to simply being destroyed by one of Picard's quantum torpedoes.
"Yesterday's Enterprise": History seems to change when the Enterprise-C appears two decades away from where it was supposed to be destroyed in battle. But the original timeline is not gone, and in the new timeline, Guinan can actually perceive that the Enterprise-C belongs to a timeline other than her own (she can even perceive some of the history of that timeline). This perception manifests itself as a disquieting sensation that something is "wrong", but that's an oversimplification. After all, how can a timeline be "wrong?" With countless timelines in existence as seen in "Parallels", why would one be more "right" or "wrong" than another? A better explanation is that Guinan perceived enough of the Enterprise-C's original timeline to know that she thought it was better than the one she was currently in. We jumped to a divergent timeline when the Enterprise-C arrived and we jumped to another divergent timeline when it departed.
Although the "many worlds" theory may have been discredited in real life, it seems to be the only way to explain Star Trek time travel as we've seen it on the show. It explains causality paradoxes in "City on the Edge of Forever" and STFC, and it also explains why time travel is not being used to solve problems, because it means that time travel doesn't really change anything. It only moves the traveller into an alternate universe where events unfold more to his liking. An interesting consequence of this explanation is that we've really been following a group of characters as they move from timeline to timeline, so we haven't stayed in a single universe throughout the series run of Star Trek.''
Link : http://www.stardestroyer.net/Empire/Essays/Trekkie.html#TimeTravel
JJ Abrams's Star Trek is set in a alternate quantum universe where the timeline has never been fixed since all the possibilities exist As simple as that.