Typically, you're more likely to see a novelization of a movie based on a pre-existing book if the book is in public domain. Any living author (or active estate) is likely to insist that the original book be reprinted instead--and probably still controls the literary rights to the property. I know of at least one instance where the studio would have preferred to put out a novelization, but this suggestion was nixed by the original author.
Meanwhile, on the flip side, as I understand it, the screenwriters have to sign off on their work being novelized. Typically, this is just a matter of course, but there was another
instance where I wanted to publish a novelization of a certain movie, but the screenwriter would not allow it. He was willing to have his original script published in book form, but would not consent to it being novelized. (I passed.)
And, come to think of it, there was a momentary hiccup on another
novelization when it was discovered that, due to a bureaucratic oversight, one of the ten
screenwriters who had worked on a certain movie had never signed the necessary paperwork to allow a novelization to be published. Fortunately, this was straightened out in a matter of days . . . and the book came out on schedule.