The two big factors go together, in my mind: the design of 5E going back to a looser, less formal and fast-paced style of play similar to 80's iterations, and the overall rise of streaming. I know for a fact that WotC wasn't thinking of the streaming breakout when they made 5E, but the design they chose really played to streaming as a medium much more than a 3.x or 4E style game would have. Combine that with a generational trend towards being open to geeky fantasy stuff, and you have a perfect storm.
Now, Critical Role is the largest single streaming group, but they aren't the only ones making a living off of that scene: they are on the pointy end of the spear, but they aren't the whole story. Adventurer Zone and Acquisitions Incorporated are also huge, and there are a multitude of people putting stuff out there, and were doing so even before Critical Role broke out big. It's a whole scene, not just CR. Though I suspect for people who don't get into stremed games, CR is sort of a condensed symbol for the entire scene?