IPv6 primarily solves a problem that most people either don't have ("I have IPv4 IPs already") or don't care about ("I don't know/care what my IP is") and it introduces a bunch of problems people didn't have before like worries over comparability with existing hardware/software (improving all the time) or even just "now I have to spend a bunch of time learning about how to correctly and securely implement this on my network" (still a problem)

Maybe one day in the distant future, IPv4 collisions/shortages will be an actual problem for most people. If that happens, those people will naturally make the switch. Until then, why would they?

It turns out a bunch of people actually like NAT. They like it so much that they pushed for solutions like NAT66 so that they can keep it even after switching to IPv6.

If IPv6 offered substantially better security/privacy, speeds, reliability, or introduced some new killer feature people didn't even know they wanted until they learned about it there wouldn't be any reason to try to force people to move to v6. Because it doesn't do any of that, and most people are happy with IPv4, they'll stick with what has been working for them.