No-one wants to use it because it's a huge pain
IPV4 is not only supported by absolutely everything, but is extremely well documented, with proxies and programs to curb its worst excesses. True, it has disadvantages, but the workarounds are generally good enough.
On the other hand IPV6's problems are legion :
Not enough services support it. This is chicken and egg, but whereas e.g. VoIP generally supports IPV6 (and should, because using it on IPV4 is nasty), e.g. Playstation consoles only really support IPV4 but require huge numbers of ports to be open[1]
IPV6 firewalling tends to be less than wonderful
IPV6 address assignment and translation is complex. Not all providers and software support all the methods, so you need to know several and how to configure them.
Complexity would be manageable if the documentation is good - it isn't. Documentation is spread around all everywhere, and isn't particularly complete, even for operating systems such as OpenBSD which have had IPV6 support for a very long time. On the other hand, the IPV4 documentation is excellent.
I've been wanting to sort a cellular failover for my network. For IPV4 it's simplicity itself, yes it relies on NAT, but basically deliver both addresses via PPPoE - one to fibre, one to a cellular router. Stick them in an OpenBSD trunk interface. Firewall to the trunk, NATing to the address dynamically.
IPV6? Well, first there's no NAT, so I need to work out how to map locally allocated addresses to a bank of external addresses, but the cellular router won't deliver a range of addresses over IPV6 using PPPoE, so I need to use another method. There's no easy guides on the Internet to do this, and this is for fun not for work, so I very quickly reach the 'lose the will to live' point.
It's on the list of things to sort in 2026, but it really should be easier than this. If I'm wrong, and there's a great website or book that really explains it properly then please let me know, but I have looked and it really is not obvious.
[1] apparently The PS5 supports IPV6 for some functions, and support was added but not really documented for later PS4 firmware. However, it only supports dual stack, not pure IPV6, and it's not very clear if this supports the PlayStation store etc, which is really all I want..