Trying to add all of SLADE's features to Zone Builder would take a lot of effort, both in terms of code and in terms of design, and the only real advantage would be a little more convenience, while bringing countless disadvantages such as added maintenance work.
Not only that, but there's already a lot of work coming up on the side of map editors: SRB2's UDMF implementation will require a separate map editor, which obviously has higher priority than trying to integrate an already-existing program that already works fine on its own.