User blog comment:Subject 16*/Assassin's Creed: Revelations - News/@comment-87.60.100.183-20110513211647/@comment-3349505-20110514182908

The developers just said that it was historically innacurate. The Crossbow is a very very old weapon that has existed for thousands of years. The real reason they didn't put it in was because of how it would have dealt with the gameplay.