TL; DR. Porque ele nunca se importou especialmente com o Star Trek original e porque ele se sentiu limitado ( como diretor) pelo universo canônico existente:
Abrams: I was, frankly, never really a fan. I never really got it. I never really cared much about it. Most of my friends who loved it were, without question, smarter than I was. I kept trying... and I couldn’t get it. I didn’t care about it. It felt stilted. It is ironic because a lot of the tone and techniques and some of the writers as well were from The Twilight Zone. When you watch it, you’d go, ‘God, there is that same kind of melodramatic vibe.’ A lot of the writers were the same writers. You’d think someone who loved The Twilight Zone as much as I did would kind of find a kinship to that show and get on board. I couldn’t do it. I enjoyed the movies that I saw, the early films, but I never looked forward to them. So, when I was mixing Mission: Impossible III… I was asked if I was interested in producing a Star Trek movie. When I said yes, it was because… I’d never thought of it, ever… but what occurred to me as I was being asked was "There’s a version of it that I could see getting interested in." And it was weird, because I couldn’t tell you what it was. I just knew that if Star Trek were done in a certain way, with an approach that somehow let me in more… I was actually being given the opportunity to at least attempt to do something that I wished had existed for me as a kid trying to get into it, which is a way in, which is an emotional way in, that was not was not about the Enterprise or Starfleet or the Prime Directive or any of that stuff, that was completely emotional. I thought if that existed I probably would have found a way in. Now, maybe I saw the wrong episodes. Maybe I wasn’t in the right frame of mind as a kid. I don’t know what it was. I have since watched a number of them and actually have actually come to really appreciate the show.
Abrams: Here’s the thing… I think the key to that was, first of all, it was one of those things that not everyone even cares about or understands the timeline of it all. The notion that when this one character, Nero, arrives in his ship, that basically the timeline is altered at that moment, so everything forward is essentially an alternative timeline. That is not to say that everything that happened in The Original Series doesn’t exist. I think, as a fan of movies and shows, if someone told me the beloved thing for me was gone, I would be upset. But we didn’t do that. We’re not saying that what happened in that original series wasn’t good, true, valid, righteous and real. Let people embrace that. We’re not rejecting that. That, to me, would have been the big mistake. We’re simply saying that, "At this moment, the very first scene in the first movie, everything that people knew of Star Trek splits off into now another timeline."