I am hardly a leading authority on such things, but it was so disputed because of it's content. There are some jurisdictions that still do not have the Apocalypse in their canon. It is also not read liturgically in the Orthodox Church. However, it goes both ways. The Church in her Wisdom employs many things from the Apocalypse in worship, such as the idea of heavenly Liturgy, but wisely IMHO, does not use it for public readings. I can't count the amount of people that have gone down very bizarre, and often heretical roads using the Revelation as a proof text. 2 Peter, and 2/3 John were also disputed as well. I think though for different reasons.