said by Samwoo:they still fall into the IPv4 address shortage, because they would still need IPv4 to route the tunnels across their internal network.
If you put the 6rd servers before the CGN NAT gateway, they can use RFC 1918 space for 6rd. Adds complexity but it can be done.
I suspect 6rd was just an emergency sort of thing. New hardware I'm sure will natively support v6 in say a year.