Because it depicts Muhammad being dragged into hell by a demon.

Respect Europe's culture and history? Pah! It is all worthless trash, jahiliyya! Eurabia Alert: "Corridors of Power in Milan, Italy: Bad Manners, Bad Intentions and More," by Roland Flamini in [1] World Politics Review, March 27 (thanks to [2] Creeping Sharia):

ISLAMIC PROTEST -- Visitors to the magnificent church of St. Petronio in Bologna are now searched by Italian police before entering because in addition to protests by Muslims offended by a depiction of Mohammed in a 14th-century fresco, there have been unsuccessful attempts to blow the painting up.

The painting of the Last Judgement, by the Renaissance artist Giovanni di Modena, shows a devil dragging the prostrate, naked body of Mohammed into hell as a heretic. Against a background of failed crusades in the Holy Land, the Catholic Church in the 14th century had no taste for religious coexistence. Now it's militant Islam that is on the offensive: sources in Bologna say security forces have stopped at least two attempts to sabotage the painting by Islamic extremists.