Bottom line, I've done a lot of work on npm over the last 4 years, and I've got some ideas about how to best take care of it in the future. Those ideas require money, and it's a lot easier to get money if you have a plan to turn it into MORE money. (This IS still a site about startups, right? ;)
If things keep expanding and growing like they have, then it's going to require more money. So, we've gotta earn our keep to be sustainable, and the way to do that is to provide stuff people will pay for.
Also, I like to make money. It's fun, and it keeps you grounded in reality, if you run it like a responsible business. So that's another nice plus of going this route. I still have full flexibility to carve things up however I want later on, and like I said, we're keeping every option on the table.