quite a few ISPs use packaged products from third parties which might be part of the problem... I am sure most ISPs network equipment has had v6 support for years, but if they have things like traffic shapers, transparent proxies, load balancers, content delivery networks* etc they might not have sufficient control to simply flick the IPv6 switch to on.
In this particular instance, its likely that newer smaller ISPs might have it easier than well established ones who have all sorts of legacy gear to worry about. * feel free to ask me off list about $employer's excellent IPv6 support in our product ;-)
