Because EX fights aren't in any roulette except mentor roulette which requires battle mentor AND duty complete of every single duty in the game short of savage except current EXs which are not in it either, they get put in later when they become old. Coils, by the way, counts as savage.

Combine with the fact that NA servers have a large tendency towards people having the 'you don't pay my sub' attitude even if not explicitly stated, in content where one person refusing to do mechs because it's 'not their playstyle' or 'stop being elitist we don't have to be perfect' can result in the entire group being unable to clear because that person is getting them killed. Combine further with 'duty complete' allowing unsync to count, so even if you do finally get a few mentors dragged in...there is a high chance they may have no idea what they're doing or immediately leave because nobody wants to spend 90 minutes dying repeatedly with never getting a clear, with randoms they will likely never see again.
So EXs get done in party finder, where you can talk and plan beforehand, explain to people who don't want to listen that if they don't follow mechs they're getting kicked because everyone has to pull their weight in that content. End result of all this? People in DF'd EXs are predisposed to not listen to things and cause wipes, amplifying everything up to here, simply by virtue of being in DF and already showing that they don't listen to things like 'use party finder'.