The fundamental difference between sports and watching RPG live play is sports are competitive and feature a winner at the end of each session. That's their primary appeal. It's why they have an audience. Nobody would watch golf or football on TV if score wasn't kept and there was no winner declared at the end. There's a reason most sports leagues have done away with ties.I don't understand why people watch sports. Nothing is more boring than watching a sports match.
So the popularity of sports doesn't give us much insight to the popularity of shows like Critical Role.
I do think there is something generational about it. As another poster commented, lots of people in their 40s and 50s have been playing videogames most of their lives, but I'd wager the audience for watching other people play videogames on Youtube or Twitch is almost entirely under 35. I expect the audience for CR skews youngish too.
We know that younger cohorts have smaller social circles and spend considerably less time engaged in face-to-face socialization than previous generations. My sense is the appeal of these shows is community. It's a way to feel like you're hanging out with friends even if you're home alone on a Friday night.