Right, I'm familiar with that aspect of how site selection works (I got my membership to Worldcon 76 by upgrading from Advance Supporting to Attending). It's usually the cheapest way to get either a Supporting or Attending Membership, and a growing awareness of that is also a likely factor in more people participating. But my understanding is that the ability to buy an Advance Supporting Membership online is fairly recent -- I remember people talking only a few years ago about the difficulty of doing so with a credit card, rather than by mailing in a check. I tried to look into when that part of the process changed, but my quick research didn't turn up any details. It might be one of those things that varies by Worldcon.