Although IPv6 is better than IPv4 for several reasons, the uptake of IPv6 has been slow for carrier networks and ISPs. There are primarily 2 reasons this. Firstly, upgrading network infrastructure to support IPv6 is expensive, and ISPs may not see the need to do so if staying with IPv4 is better for profits.
Secondly, by using NAT, ISPs are able to get multiple devices to share 1 public IP address. This delays the issue of IPv4 addresses running out, and lowers the incentive for ISPs to make the switch to IPv6.