Gear that has more complexity could lead to balance issues with the numerous different benefits of different gear.

Gear that has more complexity could also make it less casual to get the gear that players expect you to have for savage.

And the current system works and has worked for years. People chase BiS even though there is very little you actually need it for, mainly ultimates and it's nice to kill everything from dungeons to criterion dungeons faster.

So with these 3 points, I doubt they will actually change it.