He played that role/character great. He got fired?
He probably got a severance package or WWE bought out his contract and sent him away. Hassan surpassed JBL and Triple H as the most HATED heel in the company, and that takes some doing.
Basically, Muslim PC groups like CAIR kept belly-aching about how Hassan stereotyped Muslims, even with promos to try and ramp that down like his confrontation with John Cena.
When they moved Hassan to Smackdown, it got worse as UPN got slammed with yet more complaints. Airing that footage of his minions, beating down the Undertaker and carrying Daivairi out like a slain martyr (after that London bombing) was the proverbial backbreaker.
Hassan was barred from being on Smackdown. They wrote him off by changing the script for his number 1 Contender's match with the Undertaker at the Great American Bash, which Hassan was supposed to win. Instead, Undertaker won it and powerbombed Hassan off the stage, near the TitanTron. Hassan was hauled off in an ambulance and that the last we saw of him.
That also impacted the World title match. Batista was supposed to beat JBL clean, to set up Hassan vs. Batista for the title. Instead, JBL won by DQ. Onthe next Smackdown, JBL demanded a rematch, but Undertaker was the #1 contender. So, they fought in a no-disqualification match to determine who faced Batista for the title at SummerSlam. JBL won, via outside interference by Orton (still stinging over his loss to'Taker at WrestleMania 21).
Smackdown GM, Teddy Long, banned Hassan from Smackdown, saying he could go back to Raw (which he didn't); he could go back to where he came; but, as he was concerned, Hassan could go straight to hell.