The issue has less to do with story and more to do with problems regarding the availability of the quest-giver for the Hard mode trial questline, namely, Minfilia. Initially, Ifrit HM, Garuda HM, and Titan HM weren't required for story completion and were optional instances, but 2.5 made them required in anticipation of Minifilia not being available for story reasons after completing 2.55. While they would eventually run into similar problems with Urianger and the EX versions of those trials, at that point Urianger was still going to be around after that point in the story, and they eventually went with the 'time bubble' explanation as to why Urianger is still hanging around to give out those quests even if he's not actually present per the MSQ. They haven't really made any changes to that requirement since then because there hasn't really been any need to, though it wouldn't surprise me if some of this was shifted around when they do the ARR streamlining in 5.3.

As for ease of completion, the hard mode trials are in the roulette, and while you can occasionally get the bad group that can't do mechanics, that happens in plenty of other instances as well. Most runs of them, in my experience, are quick and easy.