The reason they stopped at level 12 is simply that from the beginning of the game's design, they had decided to cover a certain range of levels, as implementing all the 20 levels of D&D abilities with the game engine would have been an immense amount of work.
As for all the reasons why playing within certain level ranges works better in tabletop games or why characters are considered gods at level 20, those are deductions that have no basis in the opinions of tabletop players and have nothing to do with the video game.
