They don't need to buy as expensive cgnat hardware if a greater proportion of traffic is ipv6. That's the rationale given by an up-and-coming ISP in Australia.
Video is by far the largest bandwidth hog that most people use, and both Google/Youtube and Netflix support IPv6 for streaming last time I checked. The more traffic that supports IPv6, both from the client- and server-end of things, the less horsepower that the CG-NAT boxes need for network processing (and for things like logging of connection mappings for mandated law enforcement purposes).
There is certainly an upfront cost of getting IPv6 going for a 'legacy' ISP, but once the infrastructure is there it can reduce the ongoing costs of CG-NAT (because IPv6 will just be part of the fixed costs that you need/have anyway to be an ISP).
