The only explicit reason given for excluding the Apocalypse of Peter from the canon is that it was not actually written by the apostle Peter. We have no record of any theological objections to the text’s inclusion in the canon in the early church. As such, it would appear the Apocalypse of Peter was considered orthodox until it was no longer canonical for the majority of Christian communities. It was thus only after the majority deemed the text non-canonical that the universalism within it was further classified as heretical.I noted the publication of a new edition of the Apocalypse of Peter by Thomas J. Kras and Tobias Nicklas here back in 2004.
It is a little confusing that there are actually two Apocalypses of Peter. The Apocalypse of Peter that is the subject of this article is a second-century work preserved in somewhat different Greek and Ethiopic recensions. There is also a Coptic Apocalypse of Peter preserved in the Nag Hammadi Library. They are completely independent works.