Incentives / Demand
Currently there is very little reason for users to demand ipv6, about the only vendor doing anything positive is microsoft who publish documentation for the xbox one which encourages you to use ipv6 for a better experience. Users are not asking for ipv6, so providers don't bother offering it.
If users were demanding ipv6, isps would start providing it or lose customers, and sites would start offering dual stack at least.
A lot of US government sites are available over ipv6, because the government demanded it... In the UK, there are no government sites available over ipv6 that i'm aware of, even the relatively new gov.uk site is ipv4-only.
Even when everything supports ipv6, many people will not bother to configure it or even explicitly disable it.
One approach would be for the likes of google and facebook (who both already fully support ipv6) to start offering new (ie beta) features over ipv6 first, and displaying warnings to users accessing services without ipv6. Having beta services available over ipv6 would result in better beta testers in the short term (people with ipv6 now are more likely to be tech savvy), and result in more users demanding ipv6 from their isp.