"Thoth", the publication of the Dutch Grand Orient, issue 4/2014 has an article of a Belgian member of the Grand Orient (irregular if you want to know). He asked the exact same question and did some investigations. His conclusion seems to be that youngsters do not want a shorttrack (a three grades in one ritual was discussed in my country in the very early 20th century and dismissed). They admire the stature and ritual, but they find in FM mostly philosophers and historians and very often combined. Youngsters look at the world differently. They don't look so much at the past, but rather see an insecure future. Therefor they don't try to hold on and defend what once was, but want to use what is in the present for the future. Also they see in the Enlightenment philosophy that is very strong in FM mostly an external idealism while they want something internal, something more for themselves; not in the way we like to see individualism nowadays by the way. Youngsters want to be able to use FM in their current worldview of technology, rapid developements, multitasking, etc. instead of keeping hearing how things were in the past; not the Christian-pained humanism, but something they can use in their own worldview.
The article is too long to reproduce, but there you have some keywords.
As for myself (39 in a few weeks and EA since a few months), I must say that I noticing that most lodges meet once a week was quite a thing. FM expects to attract people with some life-experience and then more or less asks to trade in hobbies, contacts, etc. for weekly meetings. Indeed, especially people in the age of forming families may have to postphone their Masonic lives. For the rest, I hope that FM will be an addition to my life, while it is often presented as a replacement for some things (in my country often for religion), hence, I need room for other adventures; room both within the lodge, but certainly also without. As for the 'FM method', I do most certainly not want a watered-down version. Actually, most logical would seem 'the full package' but at the individual's pace. What is the rush to become a MM? Better good than fast, right? The same with the choice for higher grades. One reason my order split off Le Droit Humain was that in LDH the 1st to 33rd grade is one process and some people didn't want the 'pressure' or 'obligation' to continue with the higher grades.
Just some thoughts.