Because it might offend Muslims, of course: who wants to get blown up for an opera? From the Violent-Intimidation-Works Department: "German opera house dumps Mozart opera depicting Mohammed," from [1] DPA, with thanks to [2] Sugiero:

Berlin- One of Germany's leading opera houses, Deutsche Oper Berlin, announced Monday that it was cancelling a controversial production because of the likelihood that it might offend Muslims. The original opera, Idomeneo by Wolfgang Amadeus Mozart, makes no reference to Islam, but director Hans Neuenfels introduced a scene to his production that depicts the decapitated heads of the Prophet Mohammed, Jesus Christ, the Buddha and the Greek god Poseidon.

It caused outrage at the premiere in 2003. The opera company said it was cancelling plans to revive the show next month after advice from security authorities in Berlin that the performances posed an "incalculable" security risk.