Basically they have used 5 high draft picks over 3 years, with near no NBA on court player development over that time. They may be on track now, but going through 3 years of having a d-league team with the cornerstone players not even suiting up is tough to swallow.
I think as Celts fans we have to remember that Sixers fans watched the team be utterly and totally mediocre for over a decade.
After they made the Finals in 2001, they won 43, 48, 33, 43, 38, 35, 40, 41, 27, 41, 35 (equivalent to 43 -- lockout), and 34 games.
During that time, the Sixers made it out of the first round just twice -- 2003 and 2012. They never made it further than the second round.
That's a loooooooong time to spend in the purgatory of the NBA middle class.
During that time, their lottery picks were Iggy @ 9, Thaddeus @ 12, Turner @ 2, and MCW @ 11.
From that perspective, it's not surprising that many 76ers fans were willing to spend a few years at the absolute bottom of the league, if it meant they'd have a very good chance of finally getting the pieces necessary to climb out of the morass of the NBA middle.
Rest assured, if the Celts ever spend 12 years winning 30-45 games and rarely making it past the first round, these forums will be a writhing mass of fans screaming for the mercy of a comprehensive tank job.