Because Star Trek have very odd copyright structure. - Franchise once where split between two legal entities - CBS (Paramount Television) and Viacom (Paramount

Because Star Trek have very odd copyright structure. - Franchise once where split between two legal entities - CBS (Paramount Television) and Viacom (Paramount Pictures). Exact details where closed, but rumors said, nearly all series where at one company, and films at other, with one exception. - Because VOY and DS9 created simultaneously, and each need lot of resources, decided to make one of those two series exception, and they created in other company than all other series (as with overall split agreement, also unknown, which exactly series belongs to which entity). So they have very large troubles (bureaucracy), in sharing 3D models. Because of this, you may notice, no one series share Enterprise model with any film. And even in VOY and DS9 created simultaneously, nearly all models are also different, except of Defiant, which appear in few series of both (and now it is legacy problem). Unfortunately, this also lead to legal troubles in re-render CG with higher resolution, because copyright forbids to make changes without written agreement on every case, where used copyrighted model which belong to other entity, and nobody want to deal with those stables. And even if use some magic technology, which just enhance resolution without use of any copyrighted sources, will have to prove to the judge, that new video appear magically, without violation of agreement. Wiki says, that in approx 2019, two studios merged again, but for so large entities, this is not means, that they immediately will behave as one. - Usually, spend from 6 months to few years to complete merger, and with covid all things done much slower than usual.