Larian may be best at that particular combat system. But that does not make that combat system the best system. Two separate issues.
Never said otherwise. Some things work better for other things. Strategy games work better with TB, hack'n'slash games are better in real time, same as fighter games. Roleplaying games kind of fall into several niches. Action-adventure, strategy, third-person shooter, first-person simulator and any other number of subgenres.
D&D is a classic rpg with a certain degree of strategy involved. How 5E works is entirely dependant on the turn-based system to take into account the action, bonus action, reaction and movement mechanics.
In order to make a game based on D&D 5E as its base it would almost have to be TB for it to even work.
People said this exact same thing about D&D 2e and yet the IE games implemented the D&D mechanics just fine. And RT works just fine in many strategy game as well.