The problem with portraying more "alien" aliens on live-action TV is the expense and complexity of the requisite make-up and special effects. This was why TOS especially was very light on truly "alien" aliens, because the ability to render them was beyond TOS's capabilities of the day. Even now, after film making has advanced so much, it's still a big ask to render truly "alien" aliens.

That said, though, I have been appreciating Trek making a more concentrated effort as of late on trying to diversify their aliens a bit more, as well as including a greater variety of aliens amongst starship crews than there had been previously.

I agree with @Scyphi. I also prefer star trek to have more of a variety of truly alien looking species, but I understand from a production perspective, and also from an acting perspective, why it's more practical to have human looking species.

Star Trek aliens are allegories of human character traits, at least the major recurring races are.

Vulcans are rational, logical but emotionless. Klingons are warlike, agressive, but honorable. Ferengi are greedy. Romulans are marchiavellian, Bajorans are religious, Cardassians are totalitarian.

They are there to teach us something about ourselfes by externalizing and exaggerating our different internal personae. To achive this, or at least to make it work best, they have to be relateable, we must be able to identify with them. Many fans liked to dress up as Klingons or Vulcans ("identify as them"), long before Cosplaying was a thing.

I think that's the main reason the authors, producers and production designers sticked with the humanoid aliens as established in TOS, even when the budgeds became much higher and CGI was available (heck, even in the animated shows most aliens are humanoid).

I think the cost and time involved in makeup is a hindrance. I also think the idea was to make them more easy to blend in somewhat