It's a very noisy question. I can speculate wildly, if that's what you want. Hrm...

• Autism correlates with geekiness correlates with internet use. Internet being the delivery mechanism of LW content (to the point where there's no other way to get into LW - unlike, say, a knitting site, which could attract people who got into knitting through an offline meme source), that's a filter. You have to be somebody who's willing to acquire online hobbies.

• LW is not a haven for dreaded "small talk". (I don't dread small talk anymore, but I used to.) Jokes and so on happen, but it's not a site where a bunch of people get flung together and Have Social Interactions: instead, there's a topic. This is probably more comfortable for autistic types. The only other online community I've become heavily involved in was one based around play-by-post D&D, which, again, is a topic.

• The design is good. LW has an inoffensive color scheme. Nothing flashes or animates at me. It doesn't change layout in a visually noticeable way often. It's the website equivalent of a comfy chair in a quiet room.

• There are several angles from which one can become a competent LWer. This allows people with various niche interests to congregate and pool skills. It's a common autistic trait to pick up special interests like that, so any community with a topic one could approach via (say) math or philosophy might attract a similar crowd. However, once someone's attracted, rationality is sufficiently applicable to miscellaneous topics that it can maintain a hold on someone whose interests change. (This is not personal speculation, since I don't think I really do the "special interest" part of my diagnosis unless you really squint.)