Because doing a quality job on ENT would be too pricey for Skiffy and Paramount would be idiots to denigrate what is, after all, still a valuable brand name by doing a hack job of it.
It's much more intelligent to do what they are doing now: re-invigorate the brand with the strategy most likely to work - fresh writing/production team, going back to the elements that the public knows (Kirk, Spock, Enterprise) - and throw boatloads of money behind it.
If Abrams hadn't come along, Star Trek
would have been revived anyway sooner or later. The big thing in Hollywood is brands, brands, brands, sequels, sequels, sequels. That's all they care about anymore. And most of the brands that can be turned into profitable sequels are either coming from comic books or old TV shows. If they'll bring back Get Smart
, of course they'll bring back Star Trek
And I doubt the Star Trek
fanbase is any bigger an issue to Paramount than the Get Smart
fanbase was. I'll bet there were dyed-in-the-wool Get Smart
fans who thought recasting the role with that guy from The Office
was the biggest abomination in human history.