The only real use-case for IPv6 is self-hosting, and that's just very niche. Other than that the arguments in favor of IPv6 turn to stuff like latency and network complexity. That's not nothing, but it won't make anyone change ISP, and so ISPs don't have to care about it.

From the ISP's POV a complex IPv4 network plus a simple IPv6 network is more work than just having a complex IPv4 network. IPv6 is optional, and IPv4 isn't.

The only way I can imagine IPv6 succeeding is if somehow big websites decide to slow things down for IPv4 users enough that they complain to their ISPs.