I think the reasons vary greatly, and I don’t think it’s just a fad, though it likely will be for some.
From my discussions with young people who find the idea attractive, and my observations, I have found several common threads.
Some seem to be attracted to what they perceive as “increased” reverence.
Some seem to prefer a “distinctive” language that is set aside for worship.
Some seem to be drawn to it as the normal reaction of teens to be for whatever their parents aren’t excited about. Others, who seem to be close to their parents, seem to be drawn due to their parent’s influence–I see this especially among homeschoolers.
Some seem to be listening to some of the charismatic younger priests who are preaching that “Vatican II is the worst thing that ever happened to the Church” and are being sucked into thinking they are the “remnant Church” that is going to save the world. Many of these are being convinced that all of the ills of the world are the result of Vatican II and that going back will restore us to the “Leave it to Beaver” world of the 50’s. I find this particular group, and their motivations, to be very disturbing as there is a sense of superiority and inability to recognize the universal Church at work that portends future schism. This, except for the occasional homeschooler, is also the only group I have ever heard use the word or concept of “orthodoxy”, as it is the buzzword that the priests use to convey the the “remnant Church” idea.
Some seem drawn to the structure and discipline that they don’t seem to find anywhere else. Knowing the parents of many of these young people, I can understand their search for some sort of identity and structure.
The key is that none of these young people I know have yet experienced the TLM, so it is all based on expectations. Whether it will ultimately meet their expectations remains to be seen.
I’m personally excited, except for the one group, with whatever motivation brings them to a greater search for God and williingness to be open to both forms, regardless of which they ultimately turn to.