It comes down to balance, it's much easier to balance content with a set number rather then balance content in which the number can fluctuate. They could make content in which 4-64 can do it but then they have to balance it for 4, 12, 24, 36, 48 and 64. I mean if you look at it like this think of it as 4 is toto, 12 is Darkhold, 24 is Ifrit, 36 is Cutter's Cry, 48 is Aurum Vale and 64 is Garuda Primal. Basically in the time it would take to produce all of the above content, they would most likely have to take that same amount of time to balance out content in which the level range is varied rather then have a set level. I mean just look at the wonderful job XI does in this aspect it's either, sure you can do it with 4-18 but it'll be really tough, 19-30 is medium but if you want to make it easier you should try bringing all 40. That's not really balance since it comes down to numbers overwhelming just like the invasion content... but even then the invasion content can be done by 3 players so the difficulty for that event wasn't even put into thought lol.

Varied level content may come later but for now you have to think of it as a time constraint, it just takes more time to balance content by amount of players then it does to have a set number. Would you really be happy if instead of all the above SE only released the Darkhold dungeon but with the ability to do it with up to 4-40 players?