Logic and gut instinct. Star Trek is a billion-dollar-a-year cash cow, and because of the nature of the split between CBS and Paramount Studios, they both had a claim (and vested interest) on that cow, so the legal eagles worked out their own Solomonesque solution and split the baby in two. CBS gets the TV part, Paramount gets the movie part.
The problem is that all the previous movies are directly tied to the television properties, making them CBS property as well and leaving Paramount with nothing of their own.
Hence, the reboot.
Now they've got their own alternate timeline to play around with and make as many explodapalooza movies as their little pea pickin' hearts desire, and they don't have to share with CBS any more than the licensing fee requires. Likewise, CBS can do whatever it wants with the established canon and doesn't have to so much as give the time of day to Paramount.