I really think Emma Rigby’s availability was a factor as to why they’re still waiting to tell the story of what happened to her. This Season being crowded as is was the main factor of course, based on their answer.
Availability certainly but I also think they just plain don’t know what happened to Ana. They brought Will over to give Rumbelle grief and because they needed to give Belle something to do in S4B, but without any sort of forethought on how to integrate Will properly or make him and his story relevant. As such, he has had almost zero interaction with anyone outside of Belle (little bit of Rumple and Robin and a few passing moments with Emma and Hook) and in during that time there has been almost less than zero indication that he was ever in WL, ever involved with someone named Ana. Even when Belle had her heart taken and then given back, they failed to make any sort of Will/Ana connection. I honestly don’t think A and E have a story that they’ve thought up and planned to explain why Ana isn’t around.