Also, in the early PF1 era, the competition was 4e, not 5e. 4e, while it outsold PF1 overall, was nowhere near the behemoth 5e is, and in particular was much less friendly to 3PP (which, in fact, was part of the reason Pathfinder even exists). So from 2009 to 2016 (when 5e got OGL:ed), Pathfinder was pretty much the only game in town for 3PP supporting someone else's game. I mean, I guess there may have been some 3PP support for things like 13th Age, and we can't forget the OSR stuff and things like that, and some people were dipping their toes into making off-brand 5e stuff as well. But Pathfinder was the safe bet.
Now, it would be interesting to see if the decline in 3PP support is related specifically to PF2, or if this was a change that was already happening since 2016. My guess would be that with both 5e OGL:ed and DM's Guild open, a lot of folks saw on what side the bread was buttered and moved over to making 5e stuff. There were probably some people sticking with Pathfinder (or in some cases making dual versions) either because they genuinely like it better or because of inertia, but PF2 probably lost most of those as well.