Short answer: Businesses and governments do not like to spend money until they REALLY need to. NAT helped to delay the exhaustion of IPv4 addresses by many years, and it has only been in the last couple of years that the push for IPv6 has moved out from experimentation to be a requirement for all new equipment.
Basically, the IPv6 is still under implementation, migration to the new protocol has not yet been completed. Therefore as and when the whole system recognizes the new protocol as a base, it is only after that that they will stop using the IPv4 as their primary communications protocol. And it will end up as a secondary protocol. For the time being, we are still stuck to it and will need to continue for quite while before full implementation takes place.