IMO it's more to do with the Network/producers wanting to play it safe, since Star Trek is a big franchise, and thus a reliable cash cow now. Either that or it's a victim of its own fame. No-one wants to be the one who ruined Star Trek, for example.

Parts of the US threatened to take the original series off the air because it was so socially progressive, and I feel like the subsequent series haven't quite lived up to that part of the legacy, because they don't want to risk much.

The most emblematic of this, I feel, is the shift to the 32nd century. There was a lot of potential there, and a lot of it was just thrown away to reset everything back to something recognisable, just with a shiny new coat of paint. You would expect them to have at least moved on from warp drives, phasers, and quantum torpedoes a millennium after the fact, or that they would be almost entirely unrecognisable.