I don't think it's difficult to understand why YHWH would have been *originally* left out of the NT books. Here are 3 important points worth keeping in mind:
1. It is noteworthy that outside of the book of Revelation, not a single NT book professes itself as being a message from God. With the exception of Revelation, there is no evidence in any of the other NT books that would suggest that their writers considered themsleves to be writing under the inspiriation of God. (Note for example, Paul's words at 1 Corinthians 7:12 which can only be truthful if he did not believe that his letter was inspired)
2. Most of the books of the NT are in the form of letters and epistles that were *read out loud* at christian gatherings at which unbelieving Jews and other members of the public would/could be present - and at a time when pronouncing the divine name out loud made one liable to the charge of blasphemy!
3. There is no scripture in the bible that commands christians to use the divine name. But they can be charged by the Jews with blasphemy if they do use it!
Let's put it all together: Imagine you are a christian living at a time when you could be accused of blasphemy for using God's name and you're writing a letter to a congregation which you know will be *read out loud*, and you have no delusions of grandeur about what you're writing being inspired by God, and there is no command that forces you to use the name of God . . . why on earth would you go out of your way to write YHWH in your letter? Why would you unecessarily run the risk of the name being pronounced out loud at the meeting, causing any pharisee-like individuals who happen to be present to charge your brothers with blasphemy? I think that is the simple explanation for the absence of the divine name in the NT. First century christians didn't have an obsessive fixation on using the name as if it was commanded, but they could be charged with blasphemy for using it. There really was no need for them to fanatically bring persecution on themselves since they were not commanded to use the name by Jesus or God.
I believe that the writers of the NT books did not consider their writings to be inspired and this is why they did not feel the need to go against the law forbidding the use of the divine name. The view that these writings are inspired is a tradition that developed after the death of the writers and has been accepted and handed down on hearsay. The evidence used to support the idea that they are inspired is basically ancient recorded hearsay coupled with the fact that these writings quote the OT and seem to be somewhat harmonious with the OT. But when you really think about it, anyone who is educated in the OT can write a book harmonizing with it and quoting from it. There is nothing miraculous or divine about that. It's simply a mind shaped by OT thinking going on to produce a literary work that is . . . surprize! surprize! . . . harmonious with the OT. Duh! The criteria used to judge the NT books as inspired could also be used to judge any modern literary work that quotes and harmonizes with the OT, as being inspired - except for the fact that it is a MODERN work.