I think the better question is, why are there 3 difficulties of Diadem? Rank 1 mobs will kill solo players on all 3 difficulties, and gatherers will be one shot by any mob regardless of which they join. The 3 modes aren't amazingly different, and the difference in drop rates isn't spectacular.

It would have been cool if diadem was an alternate leveling method, with easy designed for level 50-54, normal for 55-59, and hard exclusively for level 60. The difficulties are pretty pointless as it is now.