What explanations have scholars proposed for the rejection of the Didache from the Canon? The Didache (full text) is an early Christian document giving advice on various "practical matters" such as how to baptize, when to fast, how to celebrate the Lord's Supper, etc.   It's hard to date, but generally scholars think it was contemporary with at least parts of the NT.  And it claimed apostolic authority.

Although the Didache enjoyed high status in the second-century church, the document ultimately did not make the Canon of the New Testament.  There doesn't seem to be a documented reason for the exclusion in the writings of the Church Fathers, so I imagine it is mostly speculation on what caused its decline in popularity and ultimate failure to make the canon.   Nonetheless, what reasons have scholars cited as possible reasons for its exclusion?

(Although I do consider "the Holy Spirit didn't guide believers to preserve it" a valid reason for exclusion, that isn't what I am looking for here.)
Lee McDonald says in The Biblical Canon that the most common criteria in identifying what was and wasn't canonical for the early church was apostolicity, orthodoxy, antiquity, and use, and adaptability and inspiration were also used to a lesser extent.

David King, a UMC pastor, utilized McDonald's categories in a 2012 paper on the pericanonicity of the Didache written for a Master's-level course at Iliff School of Theology. King says that the early church would have identified shortcomings in the Didache's apostolicity, antiquity, and inspiration, and that these shortcomings were enough to exclude it from the canon:


  We can relate all of these to Didache's dependence on Matthew and Luke. If Didache is dependent on the synoptics, then it is later than they are. If it comments upon them, then it is derivative. Being also anonymous, Didache fails the test of apostolicity. It represents the work of someone just outside the apostolic age. It thus also fails the test of antiquity, though just barely. No doubt, Didache is old, probably older than some texts in the canon. Still, it represents an adaptation of and commentary on the gospels. For this very reason, one might question its inspiration. All these factors may have led to Didache being placed outside, but just outside, the canon.