As I remarked yesterday
in another thread, the difference is largely in whether things get reused. TNG and its successors saved money by reusing sets, costumes, miniatures, etc. from the movies, so that helped give things a consistent look. Similarly, the episodes that revisited the TOS era -- "Relics," "Trials," "Darkly" -- either reused actual film footage from TOS or borrowed set/prop replicas to save money, or they reused and expanded on the TOS-era set pieces they'd already built. And thus, also, the look was consistent because they were recycling existing assets -- it was for budgetary and practical reasons rather than aesthetic reasons alone.
By contrast, in the instances where nothing was reused -- TMP, and now the Kelvin movies and DSC -- things get redesigned, which is what you'd expect when you hire people whose job is to be creative. It's been a long time since the earlier productions, and they can't just reuse the old stuff anymore.
Also, of course, part of it is
who is involved, who the designers and producers are. The TNG-era shows had a lot of the same behind-the-scenes people over the years, and a few were veterans of the movies. So that provided some aesthetic continuity. Between the TNG-era stuff and the post-2009 stuff, the only art staffer in common is John Eaves.