For one they already made a deal, when they released the digital collection, and re-released the blurays of the SE in 2015. Secondly, Fox will own Star Wars forever, so any OOT release will need to involve Fox today, in 2020 or at any other time until the copyright runs out around 2075.
It isn’t even really a deal so much as it is that Disney would finance the restoration and wouldn’t want to release it when Fox is getting the cut on all three films.