Then you wouldn't be the only one. I would be in the same position, and so would many others. Dark Lord is the first boss, and involved in almost every event, so there are many who have all his girls. As far as anyone can tell, it's bad luck. The general consensus is that droprates are between 0,3% and 1%. With 500 rolls and the most generous estimation of 1% droprate, that would still be around 0,65% chance of you not getting it. If we take the least generous estimation of 0,3%, that would put you at 22% chance of not getting the drop. None of those chances would make it unreasonably unlikely for someone to not get the drops.