IPv6 is just not widely used, so when you do use it, you stick out like a sore thumb. Think like a bayesian: for Google, it's easy to just block whole /32s of IPv6 space.
Yeah, and then you recycle the IP back into the pool for the next guy to work with. An operation I know of was getting 6+ million SERPs a day, budget for proxies was hundreds of thousands a year.
