It was pretty clear that for every actual angel in the Valley there were another 10 who could be good angels, but hadn't tried it because it seemed like a schlep. I know because I was one myself. And yet angels are the limiting reagent in the Valley. So it seemed like a case where with a comparatively small amount of effort we could have a big effect on the world.

Sort of like YC itself, actually. The idea behind YC is that there are 10x more people who could found startups than do, so by applying a little help at the hardest point, you can significantly increase the number of startups that get started. Things like the series AA paperwork and Angelconf are that for angels.