UPDATE: IPv6 load balancer endpoints are now GA.1 However, we continue to work on instance-level v6 addressing. Since I believe that at least some votes on this issue are for v6 all the way to the instance, I am leaving this issue open.Adam Ierymenko commented
IPv6 through to compute engine is critical for things other than just HTTP. The entire Internet is not HTTP. There's VoIP, P2P protocols, game protocols, DNS, and numerous other things. There are also protocols that want to see the inbound IP of connections and load balancers hide that.