IPv6 addresses are ugly and hard to memorize. IPv4 addresses are pretty and easier to memorize. That's about the end of the discussion as to why it's basically a failure.
It's so ingrained in you you don't even consider it a use of ipv4. 192.168.1.x is natural and pretty. ipv6 is a horror show and incompatible with the way that we think and remember things.
"Ah, crap, avahi isn't working, but fortunately I remember that that computer is blablabla.34." The IPv6 equivalent of that is like "fd12:3456:7891::1". Absolute non-starter.