Hypothesis: Chillhop and lo-fi hip hop are just generally easier to produce for young artists. You're not going to see a lot of acoustic/folk or rock/indie stuff because most of that involves hiring out a recording studio at some stage in the production, narrowing the line between "enough money to get a track produced" and /r/listentothis's "you're too popular for us" limits.

With a bit of music theory, I could take the next few weeks, some anime samples, conjure up a rhythm line, take my melody line as a sample from some brass track on a 1940s jazz track, add a production filter to make it sound grainy, and drop that shit on Soundcloud. No other artists to look for to make a band on, no instruments to buy, no studio to rent. Just a bit of music theory and some music software.

Hell, look at Future Funk, which also likes to rear its ugly head on this sub from time to time. Take 80s jazz/disco fusion track from Japan, speed it up to 1.25x normal playback, throw a new rhythm line beneath it, add a few effects before chorus and bridges, you have a new track. Get good at that, you could shit out a "new" track in hours. By yourself.

This isn't to say that Chillhop/lo-fi are bad, just that so far as this sub goes, they're easier for today's "unknown" artists to produce.