Because it might offend Muslims, of course. From Index, with thanks to Susan:
A sculpture by German artist Gregor Schneider was banned from the 51st Venice Biennale because the event’s organisers said it might be offensive to Muslims.
The sculpture was a 15-metre-high cube covered in black fabric modelled after the Ka’ba in Mecca and was set to be displayed in St. Mark’s Square. A spokesman for the Venetian arts authority said there was a danger that Muslims would feel provoked by the work, heightening the risk of the city being vulnerable to terrorist attacks. Schneider insisted that the artwork was not meant as a provocation. He said he wanted to symbolise a connection between European and Arab cultures.
The organisers tried to place the sculpture at another site, but finally decided not to display it at all. Instead, Schneider showed a video explaining his failed attempt.