There's a lot of things keeping Paradigm from coming up:
  1. Its a lv80 Alliance Raid and the final one in the series IIRC. Unless someone has the previous 2 done and this one also unlocked they won't get it. Which leads to
  2. outside of CT, none of the other Alliance Raids are mandatory. So if you want to never unlock them, you can (until CT was required in 5.3 you could actually do the opposite but I digress). For instance I have one character who has yet to unlock Void Ark but has the others (save for the current tier because they haven't gotten there in MSQ).
  3. If someone is leveling and the character isn't 80+ they won't get it in roulette.
  4. People don't queue for Alliance Raids. (I stopped after getting CT too much).
  5. Free trial players can only get up to HW. So they don't even have access to do it at all.
  6. Ilvl cheese. Its not as big as the previous factors but it is one.

And all it takes is for ONE PERSON out of 24 to apply to one of them, if not multiple.

All that said, I've been able to get into Yorha Raids fine on my alts. What times are you queueing? That may make the above worse.