From my perspective lounge operators view Priority Pass as marginal revenue for when not otherwise busy. Airports that sponsor a lounge (e.g. Escape in Reno/Oakland/etc.) want to attract international traffic, independent lounge operators (e.g. Club America at MIA) have their own prerogatives, and financial institution-connected lounges like the National Bank lounge in Montreal have their own business models. When PP customers reduce quality for the core business, it is in the best interest of the operator to restrict access.

Alaska Airlines famously denies Priority Pass entry when the lounge is busy. Other lounge operators have hours open when they won't allow PP customers, e.g. a bunch of AF lounges. The caveat: once you are granted access, they won't send a bouncer to kick you out. (Unless you're otherwise misbehaving.)

PP can be a great deal---but don't assume that because you hold the black and gold card you're guaranteed access.