For me, it's this content overall, require consistency and the ffxiv pug is consistently inconsistent. It would be too much coordination to do on the spot when you enter the FT. For example who is alliance 1? Who is alliance B? Who is Alliance C? etc etc.
Then you have then phantom jobs. Who are going to be your time mages? WHo are your knights? Chemists, Bards, Rangers, Thieves, Geos, etc etc for the required jobs and making sure you have back ups for them in case something unexpected or they lose lives?
Do they know all the nuances about the fights? Do you have the personal responsibility to count? (If it's anything that Ivalice Lighthouse has taught me, it's that it not consistent even with the bonuses.)
Do you have an interrupt order for the lockwards to get all that extra sanguine?
Forked Tower has way too many variables for it be reliably cleared by pugs and to try and do it 100 times for the achievement? Is it any surprise people defaulted to discord?