SE made this game AST only, why?

[Paladinleeds]

You fail to mention how paladin can just as well do content as the other tank roles, creating just a bit of a false scenario, or at least a misleading one. It's not that paladin needed it, but content was certainly easier on other jobs. Yet, it was nowhere near impossible to do it on a job that would require better resource management or simply higher execution to do it.

And here is the crux of their argument. The playerbase will generally slide towards the path of least resistance. If it's easier to do it with a different job, they will push people towards that job. Whereas the JP players likely accept the jobs people come with and work with them to get the clear, generally speaking NA players will ask for you to either change to that specific job (say DRK instead of PLD for Heavensward), or you won't get to go. Again, path of least resistance.