MSQ

[Vidu]

Exactly!

So why are all the other dungeons required as part of the MSQ?

Because they're telling part of the story - the parts where you go to a certain place and fight whatever danger awaits you there. Something very, very common in RPGs in general and the FF-series as a whole.
FFXIV isnt an interactive movie, Walking Simulator or some adventure, its an RPG with an action-gameplay and that happens mostly in dungeons and is integrated into the story.

But aren't included in the roulette if they are important?

Because they're included in other roulettes.
Those two specific dungeons in the MSQ-roulette are a special case - some would call them a mistake that SE thankfully never repeated. They take 2-3 times the amount of time a normal dungeon takes and are loaded with cutscenes, making them pretty boring and annoying to do after the first (maybe second) time. All other dungeons focus more on the actual gameplay, while those two 8-man-dungeons have a lot of time thats spent doing... nothing but watching a cutscene. There have been lots of complains about these dungeons from both sides - veterans and new players - often in regards to people just bailing if they get into the cutscene-heaver one of them. A problem that would only increase if someone ends up in Castrum instead of one of the 4-man-dungeons - lots of people would simply bail and hope for a shorter dungeon for their next queue instead.

Seperating these two dungeons into their own roulette (and tacking a heavty reward on it) was the best thing they could do.

All other dungeons are in their respective roulettes - leveling, level 50/60/70, level 80. Something that seems quite sensible to me, since it gives you a little bit of an idea of what to expect if you queue for one of the roulettes.