IPv6 just tried to do too much so it failed at everything. Putting letters in IP addresses made it near impossible to remember what your network settings were supposed to be.
It is nothing short of a miracle that devices can even get IPv6 addresses. SLAAC was supposed to replace DHCP, but it couldn't provide DNS server addresses. DHCPv6 was introduced to replace SLAAC, but this time they forgot to add a way to communicate a default route. This lead to Cisco, Microsoft, and Google all taking completely different approaches, and the IETF helpfully blocking any efforts at cross vendor standardization because of v6 zealots.