It's a chicken-and-egg problem. ISPs have very little incentive to deploy IPv6 when not enough of their customers are demanding it. Meanwhile, people don't demand IPv6 when all of their services work just fine over IPv4.
A big drive for IPv6 adoption seems to be that some ISPs are genuinely running out of addresses. They are forced to re-engineer their network stack in order to switch to CGNAT anyways, so deploying IPv6 along with it isn't too much extra work. As long as the ISP has plenty of IPv4 space remaining, their engineers are going to have a hard time selling spending time on IPv6 to management.
A big drive for IPv6 adoption seems to be that some ISPs are genuinely running out of addresses. They are forced to re-engineer their network stack in order to switch to CGNAT anyways, so deploying IPv6 along with it isn't too much extra work. As long as the ISP has plenty of IPv4 space remaining, their engineers are going to have a hard time selling spending time on IPv6 to management.