There are two reasons you could give for this that I'm aware of: 1. Most Google services are behind a single frontend, and supporting IPv6 is easy because that

There are two reasons you could give for this that I'm aware of: 1. Most Google services are behind a single frontend, and supporting IPv6 is easy because that frontend already supports v6 and you have to do specifically stupid things in order to not get support (backends hardly care what kind of address the request originally came in over). Cluster networking is not like that; the network is the product, not just the delivery mechanism you happen to run HTTP over, and you have to do real extra work to support both. 2. Google's IPv6 SWE team was largely wound down (most of the interesting things were “done”) a bit before GCE became a thing; at that point, IPv6 was generally expected to be done by each team and not a merry band of renegades, and GCE had more than enough other things to worry about. (I worked in the IPv6 deployment team at the time, part of it in 20% time.)