[detaro]

Guessing:

It's very much a "nice to have" feature with low priority, on a product that itself is likely low priority (Pages).

Even if an IPv4-only site is slower for some subset of users (that have IPv6-native internet connections and gateways to access IPv4), that's not something GitHub is likely to care about, especially not with Pages. Pages being a tiny bit faster doesn't make them money and few people ever notice. E.g. HTTPS support for custom domains was a lot more important.