I'd be curious to see (A). what percentage of players have all the raids unlocked, and (B). what percentage of players regularly raid. I have a feeling that might account for how hard it can be to get some raids to pop. CT is obvious, it's required for the main story, so if someone wants to see the story, they HAVE to do CT. The others? Not so much.

And before someone suggests "well, just make everyone do the raids then", CT is notoriously easy, I assume partly to let the story people get through it without a fuss. I can't imagine people want all the raids being as easy as CT.