Someone asked in an email discussion group why the letter to Laodicea, mentioned in Col. 4:16, is missing from the New Testament canon. This assumes that the suggestion that it is our Ephesian letter is unfounded.
My reply mentioned that one could broaden that question to include all the documents mentioned in the NT that were written by inspired men, such as the epistle to the Corinthians mentioned in 1 Cor. 5:9 and perhaps a letter written between our 1st and 2nd Corinthians. There’s also the possibility of a “missing” letter in 3 John 9. (What have I missed?)
Suffice it to say that the Lord in his wisdom saw that these documents were unnecessary to our faith and devotion and chose not to have them preserved as a part of the NT canon.
Beyond that is speculation, whether they were subpar (though how could that be?) or redundant, or whatever the theory. Speculation may be interesting, but ultimately has no spiritual profit, so such questions are probably best left to gather dust.