That's basically the only reason bosses ever took that long. You either needed to farm gear from that raid first to progress further, needed specific gear (in games that had elemental resistances for example), it was bugged (hello Twisters) or you needed to do the most obscure things to even summon it in the first place.
The bosses that took this long to kill were not only gear gated but came out at a time where mmos were new so pretty much everyone was absolutely terrible at the game.
