No, I think it's the community. Writing is a lonely business. I'm lucky; I'm one of the pure introverts. Most people aren't. Most people don't like closing themselves up all alone for hours and days and weeks on end. And yet that's what writing a novel requires. Part of the fun of a writing group is that it kind of qualifies as writing activity, but you don't have to be alone. So along comes Nanowrimo with its forums and community, and suddenly you're almost writing with other people in the room. People who understand.
There's also a more mysterious thing going on. Writing is a profoundly private act. Go to any talk given by a writer and they will always be asked about their "process." How do they write? Do they outline, do they have a place they go to write, does their family help or hinder, how long does it take, how many drafts? It is artistic sausage-making, yet everyone wants to see how it's made. Yet none of the answers will, in the end, answer the question "How?"
Here's a picture; we haven't had one in a while. It's an unusual locale, the library of the WGAw building in Los Angeles, looking out at the intersection of Fairfax and 3rd. Across the street is the Ross that blew up in the Summer of the Methane Leaks. I took this picture at the start of the first ever 24 Hour Screenwriting Marathon. A bunch of us locked up at noon one Saturday and tried to finish screenplays. I was one of the few who got a rough draft out of it. But a good time was had by all.
Look! Writers writing! I still don't know how they do it.