We’ll, if Zelena cast the curse, maybe she needed somebody to forget something in particular. For example, maybe in the list year, Z put that kissing curse on Robin, and Robin told Regina. But now, in present day, Regina and Robin have no memory of even being close in EF. Maybe, tomorrow Regina will try to poof something, and poof turns to Pffffft, no magic for Regina.
Or,
If Regina cast the curse, possibly to get closer to the savior?
(Zelena is weird)
And in order to get everything Regina has, why go all the way back to Eva? She shoulda found a shortcut.