You know of the scheduling process and knowledge of cast discussions as much as I do. Makes your finger wagging needlessly aggressive.
The only person I'm outright negatively judging is one, which I won't name. Others I know from various Roll20 shows which personally, aren't entertaining. With the exception of John Harper, I've known the others in some capacity before they were a guest on a one shot.
John Harper, Austin Walker, and Jerry Holkins are neither YTers or streamers, and as far as I know, Roll20 pays cast some monetary value (because it's certainly not going into making their lfg system any better). That makes some of the guests coworkers in some capacity. Does that mean none of those mentioned can't? No.. but It's almost always either people who we've seen before on RP shows or Adam's pool of friends.
I personally think there only really needs to be an interest in RPing for 3-4 hours. It is a one shot after all. I don't doubt though there are just some subjects to me that I personally would be, for lack of a better word, repulsed by, purely on theme alone (Traditional Sports for example), so the same might be said for other people.
Zeke, Bikeman, a mekeish character in Giantwaffle. Someone who constantly gets mentioned and is practically in a wrestling-ish type persona everytime he streams is DrDisrespect - someone who I've never watched before. I'm unsure the decision behind an all-female cast but... whatever.