I remember back in the 3.5 days, there was an article by someone -- the name Warren is in my head -- that talked about the main types of RPG players, and I think it's relevant here.
They were, to the best of my recollection, Storytellers, Power Gamers, Casual Players, and Completionists.
Storytellers, it should be obvious just want a good story, and they want to help with that. TTRPG and PBP both satisfy this.
Power Gamers can likewise be satisfied with PBP. In fact, they're probably more satisfied, because they have the time to strategize more here than in in-person settings.
Completionists like world exploration, side quests, investigation, finding lore. Again, that's probably easier here than in-person.
The Casual Player, (not my term, just the one the article used; I'd call them, social players if it were my choice) which used to be sort of a minority, just likes hanging out, tend to be lurkers, sort of. They're there to make jokes, have fun, and if the medium of that fun is an RPG, then they will do that too, but they're really there for the pizza. (I'm simplifying, of course.)
Everyone is a couple of these, usually. The thing is, if you are in the social category, you will find it very difficult to be satisfied in PBP. And while the social player used to be a minority (and maybe even a bit rare), it's a lot more common now. At the same time, a major piece of media that introduces people to the scene is the Actual Play which highlights the social aspect. That combination makes it hard to get a sizable portion of prospective players into PBP.
Discord is not really the same as PBP, as you can voice chat or have side discussions in real-time at will. That makes it friendlier for social players. That doesn't really happen in PBP.