From what I read, Prince was better at finishing projects before shelving them for later release than MJ was. MJ had a few songs that he got pretty much complete and then later had released ("Come Together", "Earth Song", probably most if not all of the Anthology tracks) but he never had full albums -- as in a collection of songs meant to go on the same record -- just sitting in the Vault collecting dust. After a project's deadline was missed or it was released, MJ just moved on from what I've read, maybe taking a track or two out for the next project.
I think it comes from Michael having an "it just comes to me and falls into my lap" approach, where he just came up with the instrumentals and lyrics for tracks all at once, which probably isn't a very common or productive way to make music (since I imagine you spend a lot of time waiting for the right inspiration). Prince likely had a more methodical and "scientific" approach like other artists, working on riffs or tunes here and there and writing a line or two of lyrics and building albums simultaneously over time.
