Is it possible that this is because the Non-Chiliasts rejected the Book of Revelation as Scripture, and only in the Book of Revelation is the 1000 years prophesied. It was after all the last book written and many refused to acknowledge it as part of the scriptures. Once it was accepted, the chiliasm therein had also to be accepted --- unless they were disciples of Origen's allegorical readings of scripture.
that is certainly a theoretical possibility. However, if it were a canonics issue would you not expect Irenaeus to address THAT as the centerpiece of his argument, rather than ASSUMING they were both seeking an explanation of the 1000 year pericope in Rev 20? He argues from the position that both are aware of the 1000 years issue, rather than attempting to tell them the reasons why Revelation should be accepted (It was written by John, John was an apostle and a real holy man, He was close to the Lord....., the stuff you would expect if the book's authority was in doubt). We find so much support for Revelation in OTHER church fathers (all you have to do is turn to the index if you have a copy of the ante-nicene fathers to see the quotes), that I really don't think the issue is one of canonics. Interestingly enough, that issue did not really arise until later.
DoP