2 Peter claims to be written by the apostle Peter:
Simon Peter, a servant and an apostle of Jesus Christ, to them that have obtained like precious faith with us through the righteousness of God and our Saviour Jesus Christ: (2 Peter 1:1)
And later in the same chapter:
17 For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased.
18 And this voice which came from heaven we heard, when we were with him in the holy mount.
The internal claims for authorship are very strong & unambiguous. It regularly shows up in manuscripts together with 1 Peter (see here), which had strong external attestation from very early on.
Why was the authorship doubted then and why is it doubted now? What evidence suggests that Peter was not the author?
Notes offered in hopes of promoting robust responses:
To give a fair shake to those who come down on the other side of the authorship debate and wish to offer a counterpoint, I've posed the mirror image of this question here: Authorship of 2 Peter - evidence in favor of Peter
I am not just asking about vocabulary and hapax legomena as this question is, though I don't mind if answers build a portion of their case on vocabulary.
I am not asking if Peter had the requisite level of literacy to write a letter--the practice of using an amanuensis in antiquity isn't really in dispute (see further discussion on scribes here).