Time isn't the only factor for adoption. Between the adoption of IPv4 and IPv6, the networking stack shifted away from network companies like Novell to the OSes like Windows, which delayed IPv6 support until Vista.

When IPv4 was adopted, the networking industry was a competitive space. When IPv6 came around, it was becoming stagnant, much like Internet Explorer. It wasn't until Windows Vista that IPv6 became an option, Windows 7 for professionals to consider it, and another few years later for it to actually deployable in a secure manner (and that's still questionable).

Most IT support and developers can even play with IPv6 during the early 2000s because our operating systems and network stacks didn't support it. Meanwhile, there was a boom of Internet connected devices that only supported IPv4. There are a few other things that affected adoption, but it really was a pretty bad time for IPv6 migration. It's a little better now, but "better" still isn't very good.