The first and biggest reason is network effects. If nobody else is using IPv6, then there’s little benefit to switching. Corporations and governments are always resource constrained and there’s not really a business case for switching just because it might be necessary someday. Being that corporations and governments are the ones providing the vast majority of services on the internet, that means there’s not as much reason for switching for end users as well.

Part of the reason that switching didn’t end up being necessary was that NAT did a really good job of solving the address problem. NAT let you have an internal network behind a single IP address which vastly reduced the amount of public IPs necessary. Even most people in tech probably wouldn’t realize that the concept of public vs internal network didn’t exist back in the day. Everything connected to the Internet had a publically reachable IP. When even that wasn’t enough, they just did it again and had CGNAT, carrier grade NAT. Now you could have multiple households and businesses behind a single IP.

The final reason is that IPv6 is just different from IPv4. Things like router advertisements and SLAAC require you to learn new things and rethink how you do things. What doesn’t help matters is that until you get everyone switched over to IPv6, you still have to support IPv4 in some way. Dual stack, where you run both IPv4 and IPv6 in parallel, is the most compatible way, but it’s also the most complicated to administrate. There are ways for an IPv6-only network to communicate with IPv4 clients, such as NAT64, but they each have their own caveats. These differences can result in small annoyances to administrators that add up, such as like you mentioned that IPv6 addresses are just not as memorable as IPv4. The natural fix for this is DNS. But like I said, that requires you to rethink your network. DNS goes from being a nice to have to being essential even for small networks.

Despite all these issues, IPv6 adoption is still marching along. France, India, and Germany have some of the highest rates of IPv6 capable devices, at 83.97%, 78.81%, and 74.30%. (IPv6 Adoption) Globally, we’re sitting around just under 50%. (Google IPv6 Tracker) Some sectors also have higher adoption rates. Mobile carriers are mostly IPv6-only these days. A lot of residential routers also ship with IPv6 enabled.