so there's a post going around about how mark most likely resonated with iron lung because it involves space (which he loves) and the ocean (which he hates) and how that seemed to speak to him enough to make a movie.
but i think there's another factor that might have had some influence too.
mark had just finished writing, filming, and releasing 'in space with markiplier' where his character fought like hell and lived countless lifetimes to try to save the universe. a universe where slowly, one by one, the stars were going out as the life in the universe slipped away.
and a similar experience happens in iron lung too, where all the stars and all life slipped away.
I like to think that at least in some capacity, those similarities helped spark his interest and investment inti iron lung