It might be more precise to say something like: we had the SwiftUI team available to give us regular feedback guiding the design of "willSet" observation, and no similar source of day-to-day feedback for "didSet" style observation. We'd prefer not to land a feature and bake it into API without getting that sort of experience first if we can avoid it. We know that there are use cases people have in mind, and we could certainly mock up small examples ourselves, but that's not the same as having a team building a whole system on top of it and giving immediate feedback when it falls short.
It's also really important to be clear that having done one now does not preclude doing the other in the future. Observable is an new feature, and as with all features in Swift we expect it to continue to grow and develop in the years to come, and encourage the community to help provide examples to guide that growth.