Marvin died before actor's contracts had provision for things like video and DVD. Well, maybe his last film or two, but not the early ones. Lots of lawsuits right now trying to correct that. No, the reason so many good movies took a long time to make it to DVD is confusion about who owns the rights. When video popped up in the late 70s, all sorts of fly-by-night companies bought video rights to huge numbers of films. Then the companies disappeared without ever transferring their films to video and the rights remained in limbo. Some of this is still being sorted out.
Concerning "Monte Walsh," I once spoke with Jack Schaefer and asked him if somebody in casting was displaying a sense of humor, casting Jack Palance as a good guy in MW after making him one of film's most memorable villains in "Shane." He told me Palance had asked him why he killed him off in both stories. Schaefer explained to Palance that Chet hadn't died in the novel, so he should blame the screenwriter.