http://www.dailymail.co.uk/news/article-3002530/Another-blow-Christianity-civilisation-ISIS-destroy-4th-Century-Mar-Benham-monastery-Iraq.html
Some Muslim scholars claims that the Jews and Catholics took ideas from the Koran in order to write the Talmud and the Bible. Of course, we know this is bogus history. But, in order to prove this point, ancient Catholic sites are being systematically destroyed to remove evidence of the true history, that Mohammed got his ideas from himself, the Talmud and the Bible.
If one destroys physical evidence of truth, it is much harder to convert people to Catholicism, and much easier to convert people to Islam.
Simple. Blow up the monasteries. Destroy Jewish and Christian sites that defy revisionist history.