It's created gimmick after gimmick of fake Jeans for 12 years in efforts to rescue the brand, and often treated Emma as Jean with a bleach job.
Especially since Emma seemed to be used a LOT more outside of the X-Books than Jean did when she was alive. And then you had her and Scott's romance (Which negates Jean coming back to romance him and serve as his lover AGAIN!).
In regards to fake Jean's, franchises do that all the time, and there had to be a reason that no one brought back the real Jean (Up until this point.). They also did the "replacement" Jean after her first death (Rachel Summers, Maddy.). The characters have pretty much moved on, but of course some fans and writers can't let go.