Putting on my game-developer/designer hat for a moment...

I strongly suspect that if you just add all the capped dungeons to a single roulette, people do not actually run that roulette more often. Meaning that as the expansion ages, the endgame dungeons will fill more slowly for folks just now getting there because you're spreading (hypothetically) the same number of roulettes being run across more dungeons. By splitting it into two -- the level 90 capstone and the current expert -- you can double-dip on the people who routinely run all their roulettes daily, using them to fill out the party for two runs of the endgame dungeons, rather than just one.

Moreover, for folks that don't raid, the expert roulette being limited to the most recent two dungeons keeps some focus on the dungeons likely to drop more useful gear for those folks, meaning they can run those dungeons more readily (as there's a separate pool dedicated to filling parties out for specifically those dungeons).

I get tired of the same two dungeons in Expert roulette myself as an expansion ages, but I can absolutely see reasons for this from a design standpoint.