From Google:
To qualify for Google over IPv6, your network must have good IPv6 connectivity to Google. Multiple direct interconnections are preferred, but a direct peering with multiple backup routes through transit or multiple reliable transit connections may be acceptable. Your network must provide and support production-quality IPv6 networking and provide access to a substantial number of IPv6 users. Additionally, because IPv6 problems with users' connections can cause users to become unable to access Google if Google over IPv6 is enabled, we expect you to troubleshoot any IPv6 connection problems that arise in your or your users' networks.
Simply said, some networks may have borked IPv6 which would mean that users will be unable to access Google. I can understand that they're doing this before rolling it out to everyone. Maybe there could be something like OpenDNS for IPv6 so that more advanced users have a choice?