I think the main reason is because increasing the difficulty of current elite content (especially heroic elite) makes XP more difficult to obtain. And I believe the prospect of forcing players to run hard when they have gotten used to running elite upsets a lot of folks.
That's my guess as to why they've backtracked out of making difficulty changes retroactive via nerfing champions. Introducing a new killer DM mode wouldn't upset players like making elite harder in existing content would.
Making new content harder going forward is fine though since players have not yet gotten used to that content. Players should be fine with new content being difficult. It's being forced to run existing content on lower difficulties that they've gotten used to stomping through on elite that upsets them. Same reasoning goes for a new killer DM difficulty. It won't upset players since they have no established difficulty expectation for that setting.
Can't give players candy then take it away from them. However, if they never got that candy in the first place they won't get upset when it's not there.