For me, it's that credit card processors take 3% of my incoming revenues, and then visa sucks up another 3% of most outgoing payments, not to mention taking a non-trivial amount when exchanging between currencies, etc.
Nerd appeal is one part but there's also a gold rush mentality among part of the HN audience and Bitcoin feeds into that.
Because posts like this continually keep it on our minds? Plus, it's a hell of an experiment and fun to keep track of.
My 2c: HN is full of geeks. Bitcoin appeals to geeks.