Because tobi can't be:
Who tobi is, is pretty much bound to fail imo. Midara was the last great badguy of the series. Everyone else is either dead for good or has been re-introduced currently.
I mean there is absolutely no one from the naruto legends left who would make a good badguy. The fact that he has a sharingan/rinnengan is pointless since eye-surgery is somehow extremely common.
*shrugs* Meh. I just hope kishimoto doesn't commit author suicide and name tobi someone pathetic or some average ninja with a pathetic grudge.