The reasons are manifold.
- His insistence (as outlined in Summorum Pontificum) that the Novus Ordo Missae and the Traditional Latin Mass are both two different forms of the same Roman Rite, even though liturgically and doctrinally, they profess two different religions.
- As noted in the CDF document "RESPONSES TO SOME QUESTIONS REGARDING CERTAIN ASPECTS OF THE DOCTRINE ON THE CHURCH" (which Benedict XVI explicitly approved and ordered to be published), he still defended and maintained that Lumen Gentium is orthodox, even though it changed Catholic doctrine with regards to the unity of the Church (namely, that the Church of Christ "subsists in" the Catholic Church, with the various Orthodox and Protestant sects treated as having "partial" communion; prior to Vatican II, the Catholic Church was always uniquely identified as the sole Church of Christ, and the various Orthodox and Protestant sects were properly called schismatics and heretics who did not possess the true faith).
- His scandalous and active participation in Islamic prayer at the Blue Mosque in Turkey in 2006 (having removed his shoes, received instruction on the basics of Muslim prayer, and turned towards Mecca in a posture of Islamic prayer, making no public signs that he was engaging in a Catholic prayer whatsoever; this action by itself in a saner era would have him branded as suspect of heresy, if not of tacit apostasy):
- On that note, his scandalous participation in the Assisi interreligious meeting in 2011 (following the example of his predecessor John Paul II from 25 years prior), which would have also been deemed an example of tacit apostasy. For a true Pope to pray alongside pagans, heretics, infidels, and schismatics — at an event not dedicated to their conversion to the one true religion, but one dedicated to 'praying for peace', as though peace is possible without Christ — would have been an unthinkable display of religious indifferentism, which has been condemned strenuously by numerous Popes.
I could go on.