Routers have to go through qualification tests-- sometimes very difficult ones, in third party labs. I'm sure there are a lot of features Cisco would like to drop from IPv4 and IPv6, but guess what: they can't. Not if they want to call their equpiment compatible.
You are focusing on the wrong end of the problem completely. Big equipment vendors love new standards, especially if they're complicated and difficult to implement. It creates churn, which means more purchase orders, which means more money.
The problem is that IPv6, as designed, is as useless as a screen door on a submarine until the magic moment arrives-- the IPv6 rapture, if you will. And ISPs are focused on Q4.