I think this is the answer. A PhD becomes it's own monster. Several years dedicated to a single project, it becomes a personal part of your identity. The thought of allowing that to stand up against scrutiny to other professors is a tough thing, and back then there weren't such limits on timeframes. As the mental block of submitting it becomes higher, and time moves on, and you get busy with a new job, family life, it gets harder and harder to do that.