Did the Dynamite Kid refusing to drop the tag team belts to Sheik/Volkoff cause Vince to change where he was going with Danny Davis? Slick appears right before Davis starts becoming noticeable as a referee. Monsoon insinuated bribes were paid to Danny Davis. Perhaps Slick is paying off Davis while also laundering money for Bobby Heenan by selling Hercules at an inflated price, on camera?
Davis as a corrupt official bought to help the 'Evil Foreigners' and Slick win the tag team titles appears to be more consistent for Vince's vision than joining Jimmy Hart's stable. Also, his heat decreased simultaneously to HTM's increasing while in the same stable.
Far as I know joining the Hart Foundation was always the long game for Davis. Plus, let's face it, evil referee turned wrestler always had a shelf life.