What do you replace it with?
This may not be the biggest or best reason but it’s the most honest. If you intend on doing a Star Trek movie in the next two-three years, and it’s not the Kelvin franchise, what do you replace it with? Sure, you could stay in the Kelvin franchise and move on to remakes of The Next Generation, Voyager or Deep Space Nine. That, however, just gives you more of the same issues that you’re already dealing with.
Usually, the film franchise embraced the television series for their films. At the end of the ’70s, The Original Series got its franchise. That went on through the start of the Star Trek: The Next Generation television series. Three years after the end of the original Kirk and crew’s time with the franchise, the Next Generation crew and it’s captain Jean-Luc Picard took over the film duties.
Voyager, Deeps Space Nine, and Enterprise didn’t get their shots after that. Those shows have been off the air for too long to switch to one of them now, so what do you do? Do fans really want a Star Trek: Discovery film franchise? Even if we subscribe to the notion that all modern Star Trek fans love Discovery, Discovery is still not as highly viewed as the Next Generation. that’s simply because of the paywall that Discovery is behind and the fact the Next Generation series was on free T.V.
So if you don’t have a viable replacement, why move on? There’s no way a new film franchise would be launched with new, original characters. Not in this generation of film making. It would have to be an established property and currently, there isn’t one that has the ideal or the support of a film franchise.