So, I put the question of Critical Role to my de-facto god-daughter. She's in school at the moment, but her dad (who plays in my regular table) gave his $0.02 from watching a bit of the show, and his daughter...
1) "Matt Mercer is a genius". He knows his game, does great world-building, sets scenes and tones better than pretty much anyone out there. That really can draw someone in.
2) The players are all professional actors. They depict characters better than pretty much anyone out there.
2a) Stunt casting - the show is made in LA, so they get interesting guests on occasion.
2b) Serialized fiction - ever been a fan of any TV drama? Same deal. This replaces normal television stories for many viewers. They become fans of this instead of becoming fans of Star Trek.
3) Fan community - the fan community that has grown around the show are supporting, sharing, and like to engage with each other. They generate cosplay, and how-to videos, have active, engagine, and creative social media circles, do real-world meetups, charity events, and so on. We, here on EN World, get to gether to... argue, mostly. They get together to do stuff.