There is a huge gap between being able to do a card trick and/or sleight and being able to described it accurately in print. (Harry Lorayne being that rare combination of talents.) Scarne may not have had that skill finding it more profitable to have others do the writing for him.
He may not have been interested in writing a book for magicians since the market was so small back then. Scarne may have thought his time better spent developing premiums for business, doing his consulting, or pushing the games he created while running John Scarne Games, Inc.
for a few details.