A big part of it is that even when the games are really good, like Budokai 3 or the Wii Tenkaichi games, they still appeal primarily to Dragonball fans, and anyone who isn't already a fan is going to rate them much lower by default because they have 0 interest in the characters or the premise. It's like if someone who thought Mario and Kirby were stupid kids stuff reviewed Super Smash Brothers. He's going to rate it lower than the Mario or Kirby fan. So the games end up perhaps overrated by fans and underrated by everyone else.
Honestly, I'm more shocked at this game's high rating than past low ratings for good games.