YouTube played a big part in growing the fan base.
The song "Micheal in the Bathroom" drew a lot of attention.
The show was recommended to the fans of "Heathers" and "Dear Evan Hansen."
Some have joked that musicals about "teenagers who hate themselves" attract and inspire like minded fans
("Mean Girls" has joined that list.)
Most interesting to me is the rise of fan animatics. Fans will illustrate storyboards to play along with the songs. One artist has created animated cartoons of three numbers that are of very high quality. I haven't seen many musicals inspire this type of creative project.
This one is a personal favorite;
https://youtu.be/nIVuAZwHOgc
threepanelmusicals.blogspot.com