Once upon a time, briefly, American Express was the exclusive USA sponsor of Priority Pass memberships. Other banks were forced into Lounge Club, which is operated by the same company (Collinson). Lounge Club had access to only a subset of the Priority Pass network.
I have no information about why this arrangement ended, but most of the former Lounge Club sponsors are now handing out Priority Pass, and Collinson has lost one tool that could have provided a method to limit or prioritize access.
However, with Citi, Chase, US Bank etc etc all issuing ~$450 cards I doubt Collinson could roll this back simply by raising their price.