I think they tried to add Will in OUAT for audience as it was one of the favourite character of the spin-off. But people who don't see the spin-off didn't care so it has no impact. And instead of continuing what they began, they prefer to stop, saying out side of the show that the event of season 4 of the main show happens before what we see in the spin-off, but never clarify it in the main show. Because they care about the audience they make, but don't respect the audience.
