The crew was actively discouraged against working with IATSE, and lower budget productions aren't really tracked by the union. There are like formal and informal rules that create loopholes for indie films in the labor laws. Then, the producers sold the movie to Focus Features, a Canadian distributor outside of US union reach and influence, for $13M, stirring up controversy avout, among other things, the visual effects coordinator was paid $200 per diem on a production that took like 3 weeks or something. Unlike the Markiplier movie, profits weren't shared, and that's why there needs to be union rep.

Combined with the general trend of basically all new entertainment business models of the last decade going toward busting unions and paying actors and writers less, especially as labor movements kick off in these industries, and you can see how the success of this movie is being goosed by how much profit it generates outside of union contract agreements.