The problem is that you have to have working IPv4 in some form, because enough major sites (e.g. Twitter, Wordpress, Github, Pintrest, Vimeo, bit.ly, TikTok, Tumblr, the New York Times, but also mozilla.org and Archive.org) don't support IPv6. [1]
Likewise, some Linux mirrors don't support it. If I decide to try an IPv6-only server, and the first thing I see after "apt update" is network errors, I'm not very motivated to use IPv6 in the future.
Then add some problems where having IPv6 enabled randomly breaks things for one reason or another. I can either debug it forever, or turn the "internet is broken" switch (labeled "Enable IPv6") to "off" and get on with my day. That means more people that support IPv4 but not IPv6 (to be fair, this one seems to have gotten a lot better).
But ultimately, until some major, hard-to-avoid site says "we'll only be on IPv6", there is little reason for most to use it, and many reasons not to. That's why the transition takes forever.