Imagine rolling a ranger in BG3. You took Tundra as your favored Terrain and Dragon as your Favored Enemy.

Congrats, none of your class features work for all of Act 1 and prooobably all of Act 2 once we reach Baldur's Gate.

Now imagine reaching a high enough level to choose a second Favored Terrain. Say, swamp because there was one in Act 1 and that environment was hazardous. Only to later find out the rest of the game is a dungeon crawl through an Ilithid city or something.

PHB ranger was bad because its core class features were all ribbons that only had an effect under very specific circumstance. This could be worked around by a good DM who was willing to change their plans to accommodate a player. But BG3, without a human DM behind the screen, cannot be that accommodating. It is physically impossible.

Ergo PHB ranger was not an option.

As for why they didn't use Tasha's? I am guessing because Tasha's wasn't going to be out by the time BG3 hit early access and Wizards didn't want to spoil their tabletop release with a video game adaptation.