Bonus game restrictions: why are some slots completely blocked.

so this has happened to me a couple times now - i get a sweet bonus offer but when i go to play, like half the slots i actually want are just grayed out. i get why some new releases or progressives might not count, but then there’s just random old games blocked for no clear reason. just seems weird and honestly a bit annoying when you already have to follow a bunch of other rules. anybody know what goes into deciding which slots get blocked for bonuses, or if there’s a pattern to watch for?