I think it has something to do with the controlled comfort of modern life. And how even a small disruption can become unsettling.
Like in HoL, the most chilling scene isn't anything that happens beyond the door imo. It's when the book falls because the house changed size ever so slightly. I think the classic haunted house trope is at play there too - home is comfort but those stories frequently involve a move, which is inherently stressful. I remember when I bought my house..every new noise or small change was disturbing. Potentially a hidden horror lurking in the house (like a water leak).
edit: Another thing I will say is that I've noticed both HoL and Backrooms seem to act like a kind of shibboleth for a particular demographic (not even really the same demographic) and you often see this in how people write/talk about both. I think it maybe stems from how dense/unapproachable the two works are, how innocuous they seem on the surface such that you really have to sink some effort to get at them.
I don't have any real proof for this, but it feels like House of Leaves inspired a lot of the people making "found footage" and "creepypasta" stuff one the internet in the 2000s and early 2010s (SCP, Marble Hornets, Slender Man), and then that stuff came together to inspire the Backrooms.
House Of Leaves is similiar to Backrooms in a way - they represent the same kinds of horror but it's more like how Weird Fiction converged and inspired in the same way. There's a level of early slenderman there as well (in terms of a lot of the early slenderman horror being more about the horror of dreaming this entity into the real)
I'd argue SCP Foundation is probably one of the main initial examples of internet occult, the Backrooms have more in common with a few SCPs.
To go wider there's probably a convergent horror - It's the classic aspect of horror stories of the age represent subliminal fears of the age (e.g, bloodsucking vampires mean very different things across the past thousand years). I think liminal horror is a representation of a lot of our fears, so multiple different effective horrors have converged on these feelings of discomfort with spaces.
I'm not really smart enough to know what it means - probably something about modern society dissassociating people from the space, but don't know much more.