If the question is, "Why isn't IPv6 the most prominent protocol and v4 a legacy thing?" the answer is NAT and RFC1918 addresses. And NATs within NATs within NAT

If the question is, "Why isn't IPv6 the most prominent protocol and v4 a legacy thing?" the answer is NAT and RFC1918 addresses. And NATs within NATs within NATs. The only "real" v4 address you need is for your publicly facing servers. There has been no incentive to go to v6.