Too tiring is part of it, but the main reason is that NBA guards can can actually dribble and pass. This full court pressure stuff only works when your opponents have bad guards. Almost all teams could break the press. Once that happens, you end give them mismatches on the end that would probably end up in a layup. Furthermore, there are so few good players in the NBA that you take a risk of getting them in foul trouble on plays 50 feet from the basket.