Ik this is about tanks here but this statement is false if you look at healers (or maybe healers seem to be the exception to this rule idk), Ast is defacto best healer from an output standpoint, Sch is defacto worst healer from an output standpoint, Sch is played nearly 12k more than Ast is(savage anyway) and whm is more than double Ast.
https://www.fflogs.com/zone/statisti...#class=Healers
If numbers was all that people cared about Ast would be higher than both of the other two but it is not, it is dead last which begs the question why? And the simplest answer is people just don't enjoy it compared to the other two.
The difference between current healers and pld in HW is the content doesn't work against them, pld's design was a physical mitigating tank in HW then they design raid tiers that were heavy magical dmg this is what caused pld's numbers to plummet their skills were useless for a lot of things baring auto attacks heck their shield was useless for near everything non auto for Gordias. Pld suffered hard for the devs lack of foresight on making most things magical damage (something they said they decide at the last minute when designing fights so not hard to see how it slipped through) in HW, fortunately Pld design changed to allow it to be useful against magic damage had the physical limitation remained to this day I can be sure that pld wouldn't be played as much as it currently is.
It comes down to how a job functions, if all function well in a given role, people would then turn to the playstyle they enjoy the most, numbers tend to be the last thing people care about (at least for healers) for what they want to play as.