Extreme trials and the DF

[Canadane]

Any reason why put EX trials in the regular trial DF (and, correspondingly, roulette)?

Because they aren't new.
Cinder Drift and Memoria Misera are new, they are in high end. When they are replaced they will no longer be there.

That being said, where it is in the queuing system means very little for the majority of the NA/EU playerbase, they'll use Party Finder to match with people, they don't draw randoms from Duty Finder.