My guess would be that people heavy into bitcoins, such as bitcoin site webmasters, are going to be fairly anti-Google due to their perceived evilness. They might also want to promote one of the bitcoin based advertising options.
Yeah I thought about this: Most bitcoin using people will be fairly tech savvy and have good 'ad blindness'. Maybe for that reason bitcoin related ads will work better since people using bitcoin want to keep bitcoin running and will support those types of banners.
Any ideas from a webmaster and their experience with adsense vs. bitcoin related ads?