The book was [accepted into the canon at the Council of Carthage in 397 AD.](http://en.wikipedia.org/wiki/Book_of_Revelation#Canonical_history)  

It was, at the time when the canon was being constructed, [believed to be authored](http://en.wikipedia.org/wiki/Book_of_Revelation#Traditional_view) by [the Apostle John](http://en.wikipedia.org/wiki/John_the_Apostle).  Anything written by one of the disciples of Jesus tends to be held sacred.

There was some [opposition to its inclusion](http://en.wikipedia.org/wiki/Book_of_Revelation#Canonical_history).  One of the views against this was that it was one of the main books of [Montanism](http://en.wikipedia.org/wiki/Montanism), which was considered heretical at that time.  [Gregory of Nazianzus](http://en.wikipedia.org/wiki/Gregory_of_Nazianzus) argued against its inclusion due to the difficult in interpretation and the possibility of abuse.

However there was a precedent of it being included in the canon that extended back to the 2nd century.

### Summary
Given the tradition of it being included in the early precursors to the canon and its generally accepted authorship by an apostle, the book was included in the canon.