It is because most of the people don't realize what exactly makes blm a hard job, and it's not the rotation which in fact is one of the easiest. Blm is a hard job because the mastery of the encounter impacts your performance way, way more than any other job, because even though is a simple rotation, it requires a good amount of precision, and for example, being on the move without casting your Fire IV for a couple of seconds could cost your Astral Fire. Besides, 2 stacks of Triplecast and one Swift helps little to correct rotational mistakes, because you can't use them all the time, and the proper application comes also with mastery of the encounter and knowing where in the fight script you should use them.
All and all, what makes blm a hard job is not that bad, though. You only need to be at the top of your dps game when you get to see the enrage, and if you're that far, is very likely that you already mastered a good portion of the encounter mechanics, and thus, mastered blm as a whole for that fight.