Great question that I don't think the other commentors really answered. Sadly I don't really know "why" either. Second system effect? Instead of making IPv6 ju

Great question that I don't think the other commentors really answered. Sadly I don't really know "why" either. Second system effect? Instead of making IPv6 just "IPv4 with 128bit instead of 32bit address space" the designers changed a bunch of things. NDP is new and different than ARP. ICMPv6 is extended and different. MLD instead of IGMP for multicast group membership. DHCP is extended and has a bunch of other alternatives, like SLAAC. IPsec is mandatory now. All the new IPv6 protocols seem more secure, more powerful, and more efficient. So I guess that is "why". But I bet in an alternative history where backwards compatibility and the upgrade process were prioritized IPv6 would be a lot more widespread.