Mentor Roulette also requires players to complete all instances in the roulette to unlock it.

Historically people purposefully not unlocking tougher dungeons has been a problem for the game. Qarn was added to the 2.1 Main story to force people to unlock it since it was often skipped.
Expert roulette originally didn't require you to complete every instance in it but in 2.1 people would purposefully only unlock Copperbell HM and Haukke HM and skip Pharos Sirius since it was a lot harder than the other two at the time. So SE changed how the roulette worked to require all instances completed.

Leveling and Guildhest roulettes don't require you to complete everything likely because the developers want players to be able to use them to help level their first job.
Trial originally required you to complete every trial in it but that was changed presumably so people who don't have the expansion can participate in it. Also possibly due to complaints by players that disliked the Hildebrand questline and hated having to do it for the trials.