just glancing through it, i think my answer would have to be majorly based on person to person alongside what semirhage said. don't get me wrong, a lot of what gets coded into a job defines its simplicity/complexity (or lack of even either), but that's just on a coding level. human error, no matter what, always plays a factor in someone's ability to handle something. AST could be incredibly simple to someone, easier than breathing for another, someone could have the luck to pull out the *exact* card they need exactly when they need it, or it could be the exact opposite in every circumstance. what could be "easy" to play for one person is hard for another. i have a friend who used to play RDM, but switched over to DRK. the reason? because he considered RDM to be too intensive for him to focus on, and DRK was simple enough for him to figure out everything and play it. it didn't matter that it was a tank and that he would have to semi learn how to do proper tanking. it didn't really matter (at first, i think he's planning to stop cause his group is basically gaslighting him for playing DRK) that it was the "worst" tank or that it was cut down heavily from what it used to be in SB. it was a simple and easy to understand action and ability list, and that drew him towards playing it over RDM.

another factor i believe is also availability, even if it can be a minimal one compared to everything else. two of the healers in game are locked behind base content. WHM is the only job you can start out as. SCH requires you to play through to level 30 and start the Sylph portion of main ARR to even unlock (when you're going to check if they're planning on summoning ramuh or not, i think around thousand maws of toto-rak?). for AST, you need to get through the entirety of ARR, both main and post (with having to do Crystal Tower raids for good measure). that's at LEAST over 100 hours worth of things you need to do to even reach the start of HW, get into Ishgard, and unlock AST for play, not to mention leveling up to 50 from 30 so you can play it through the story. SGE is a bit less constricting, but it still requires you to go from level 1 to level 70, 20 levels under the current cap. even if you were to consider either "simple" or "easy", that's more than enough time for even a beginner healer to figure out WHM and somewhat SCHs skill levels/ceiling (however low it may be) and be more than comfortable to continue playing that. consider a player that decides they won't get SGE until they got into the expansion it was released. basically ignoring it all throughout SB and ShB until they start the EW portion of the MSQ. by then, they've most likely gotten at least one healer to 90 if they've been doing roulettes alongside MSQ. they've played through all the content from start to end with one healer and one healer alone. that's the healer they'll know the most, the easiest they can play with.