He could not self-manage his work once he had wealth and had no need to write for living. Once his editor rented cheap hotel room, removed all distractions and hovered over him until the book was done. Adams worked with Salmon of Doubt 10 years and didn't even manage the first draft.
My favorite Douglas Adams' procrastination story is when he was offered £50,000 to write Hitchhiker's Guide to the Galaxy calendar. He then procrastinate it until the deal fallen through and got paid £25,000 for doing nothing.[1]
Salmon of Doubt