The cost (in addition to those that other replies have pointed out) is also one of support. While a <10yr old device may support IPv6, it is often that they don't always perform exactly as designed. IPv4 & IPv6 co-existence is a little bit of a crapshoot at the device level.

This translates into super-large ramifications, especially for companies like Comcast--I can only imagine that they don't want to add yet another possible moving part, especially when there's almost zero benefit to the average customer.