Everyone's making money on IPv4, so there isn't the incentive just yet to really switch or invest in even supporting it. Major clouds now charge per IP, residential ISPs are starting to make having a public IP a feature they charge extra for otherwise you get CGNAT, mobile carriers don't want people to host stuff and are quite happy with CGNAT.

And then there's the implementation part where everyone seems to go out of their way to do it wrong and cause trouble. My OVH server for example assigns me a non-routed /56 and I can only use about 8 of them before their router starts ignoring the rest.

At home I have to do 6rd over PPPoE over VLAN which causes my router to not be able to do hardware accelerated routing and I lose 3/4 of my connection speed on top of the resulting tiny MTU, and it turns out IPv6 doesn't like that. And then a few days later their 6rd endpoint changes and your connection dies until restarted manually, and somehow you end up with another IPv6 block and ugh it's just so horribly broken.

I want IPv6 to work but damn, ISPs aren't making it easy to adopt it in the first place.