It's a bit of a chicken-and-egg problem. Major sites don't deploy IPv6 because "everyone has IPv4", and people don't roll out IPv6-only servers because they still want to be able to reach IPv4-only hosts.

Amazon has hopefully started a change in this mentality by charging money for every IPv4 address used. This creates a real financial incentive to deploy IPv6-only servers, which in turn means that critical services like Github and Linux mirrors are slowly forced to provide IPv6.