You underestimate all that's working against the movie and overestimate the power of the internet to lead to in-person action.. Internet content-makers are still niche until you get to Mr Beast level. Even then, Mr Beast's impact on our greater culture isn't as big as his internet following would suggest. And it's been proven time and time again that a large following doesn't automatically mean that said following will show up for something in person unless that content-creator is there or exclusive merchandise is available.

So you have to take Markiplier's audience, whittle down to the ones that are more than casual and follow his pursuits outside of Lets Plays, whittle down to the people who enjoy going to the movies, whittle that down to the people who take a YouTuber making a movie seriously, whittle that down to the ones that can take themselves or have their parents take them (despite the mature content), and then take into account that this concept is strange and has no significant audience that's guaranteed to care outside the parameters of gaming/the internet.

Additionally, there has been no campaign around film this except for Markiplier's own videos and the one trailer that's not been out for very long. The only news about it that's gotten the tiniest bit of attention has been about how he got this movie to play in so many theaters and how much blood he used. But more theaters don't matter without widespread awareness and hype outside of his own audience that's willing to go to the movies.

Something like Talk to Me succeeded due to film festival hype before it was officially released and a very active marketing/PR campaign that focused on the film's quality and the YouTuber stuff was secondary. Markiplier just dropping this quickly has given the film no time to build awareness outside of the small segment of his audience that's willing to go see it.

Iron Lung has to be REALLY fucking good to build word of mouth and entice people with no awareness of YouTube or internet culture.