It all starts and ends with Vince McMahon. He knew what Tensai was even before bringing him back, and there was no way McMahon would let him work the way he did in Japan, so of course he was going to be a bust with the fans.
Tensai did nothing wrong. He came in and worked the way WWE told him to. He is better than what he has shown, but he was given a bad gimmick and portrayed as a monster that he isn't. At least he got a good contract for all his troubles.