It is for monetary reasons. If they can't make money from their creations, it's no use for them to release them. That is why they wait for the estate to decide to release them, because if they do it themselves they will not be able to monetize with it
The only time that snippets of MJ were legally shown on any platform were Faking Michael's snippets, but for research and authorization purposes of the producers, I think they aren't winning money of them. The GYWOOM snippet from the mjcast was already on yt too, so tecnically Prince wasn't playing anything (and anyway it's the same thing as FM snippets)