Someone drove over his double and wrecked it. So he picked up the 416 Mauser 98 because he was in a spot and needed a gun fast. He liked it so much he never put it down ... for thirty some more years. I think the fact that Selby, a left hand shooter, stuck with that right hand bolt action speaks volumes about the inferior utility of a double rifle. Obviously, more shots in the gun meant more to Selby than two quick shots and a bit of nostalgia...
As @Hunter-Habib mentioned above if a double was not available to be purchased then there is no other choice but to go with a bolt.
"Inferior utility of a double rifle"? Sometimes the two quick shots are what is needed when one is being charged by an elephant at ten paces or multiple elephants. Of course, we all know from your previous posts that you can cycle and fire multiple accurate shots within a second, but not every PH is as proficient as you.