Soft knowledge would be the Six series revolvers were designed from the beginning to be center fire revolvers and Ruger wanted to get into the law enforcement market. Ruger already had the popular SA and semi auto .22's so probably did not see a need to compete with S&W in the DA .22 market.
There probably is corporate knowledge somewhere about this but those involved are probably either pretty old or gone. BUT, with Ruger, you NEVER SAY NEVER. There may be notes somewhere, but remember that was well before everything was saved on computers and the World Wide Web and people know that early record/note keeping in the company could be lacking.