Because it costs money to maintain both stacks, it costs money to replace the old ass devices/hardware that are IPv4 only (at some level -- there are plenty of

Because it costs money to maintain both stacks, it costs money to replace the old ass devices/hardware that are IPv4 only (at some level -- there are plenty of situations where the end user can't spend any reasonable amount of money to replace the item because the manufacturers won't fix the problem), and it costs money to have employees spend time to get trained up in IPv6. I'd like IPv4 to die, but I've also worked at places where it would have a significant (non-internet) international impact if you suddenly shot it in the back of the head.