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.