I think the occasional 'I'm going to kill myself, talk me out of it' posts or just the high frequency of 'am I multiple?' posts, are the main reason. But also some people just want to talk to other functional plurals without getting lots of comments which imply it's a disorder or that the person writing is the 'real one' and the others are the alters, and so on.

Mainly plural_living has a much tighter concept of on and off topic; it's discussing the day to day of living plural and the workings of functional plural operating systems, so there's a higher signal to noise ratio there for people who are just interested in that.