If WHM is the 'most popular' job, I'd hazard it's far less because of some mass appeal and people enjoying it, and more so because it's literally the only healer you can start the game with, and every other role having more options from the get go so people don't all clump up on one dps or tank the same way healers get funneled into WHM. Hell even when you've been at the game for a while and have all the jobs unlocked, there's still more tanks and *drastically* more DPS (all with varied play styles to boot), where as healers are all pretty samey once you figure out how they work so there's not a ton of reason to swap to a different one if it's something you only pull out on the rare occasion to shorten a queue anyway.