Because it's art. Not science.
I mean, yes and no. TOS might be in continuity but then ignores other things within the shows details. So, if the set dressing and uniforms are that important then I could see it standing out, and it's why TMP stands out. To your point above, it changes things for no reason except because.
But, to me, TOS is very much its own thing. It operates in a bigger galaxy, with a lot more strange aliens and a different warp scale. TNG paid lip service to it but really just acknowledged it for brief winks to the audience. It upends some of TOS' views for its own stories. TMP does the same thing, so treating it as its own thing, with the UESPA, and such, sits a lot better than some of the later shows.
The reason comes back to an idea that I think many Trek fans disagree with and that is Trek is an art, not a historical piece. So, the change comes from artists and their desires to be creative, even within an established property. Even Star Wars went far afield of this with the Prequels, because it's meant to be as much an expression of the designers ideas as it is to work within the universe.
Now, I come at this because I have several friends who are artists and like to adjust things because they can. They see things differently than I do. Do I always agree? No, not at all. But, I can accept a change because of it because it is meant as an art.