ZOS buffed stat bonuses from gear, so 5pc sets had to get nerfed. Its just a yo-yo of buffing and nerfing cycles until ZOS realizes that this game of vertical progression via resource and stat inflation is a losing strategy that just frustrates players and makes balancing tedious and frustrating.

So much of the game is based off your stats. Your total stat pools affect damage, shields, heals etc. But ZOS keeps giving sets that give bigger stat increases and more CP points that also give bigger stat increases which then increases damage, heals, and shields. So to balance that, they nerf 5pc bonuses.

One day, hopefully ZOS (and a part of the playerbase) will learn that Vertical Progression is BORING and leads to an unbalanced game that has to be constantly re-balanced with soul crushing nerfs.

5pc bonuses should add UTILITY, not just more stats. If ZOS just capped the effect stats had on heals/shields/damage after a certain point. (Lets say 35k for magicka/stamina and 45k for health) then this would mitigate the power creep caused by stat stacking and lead to less nerfs. Sets with 5pc stat bonuses (plague doctor, necropotence, draugr) would still be good for hybrid builds and we would have a lot less nerfs.