There are fundamental technical reasons for it
Looking around, good, no v6 geeks or $deities in the vicinity so I am not going to look like one of those Looney Tunes "flattened character" animations after being run over by an angry rhinoceros (let's say Fred Baker or Mark Townsley).
The fundamental technical reasons for this are as follows:
1. Most of the problems of v4 in the initial problem statement for v6 have been solved. Long ago. One way or another. v6 either breaks the solution, has incomplete and complex support requiring multiple network elements or does outright incorrect and idiotic network design assumptions which have never been fixed. Example - device configuration. v6 erroneously hands out power for this to the router(s) via router advertisements, cannot be fully configured without multiple solutions present (with accompanying complexity) and all of this has been solved long ago in DHCP. The whole v6 autoconfig in this day and age is a solution looking for a problem. Instead of admitting it, revising the protocol and obsoleting all the b***cks in it, the v6 geeks deliberately crippled DHCP so you cannot get v6 config in the v4 reply and v6 version is subtly incompatible with v4.
2. The end to end principle is idiotic in the highest order. It was a jolly good idea when the machines on the Internet were in their thousands, each had an admin and there was some responsibility involved. It started looking a bit nuts when the machine count went into the millions with virus ridden 95s joining the net. It is a complete lunacy when the machines are in the billions and more than half of that are embedded builds which get no patching. YOU DO NOT WANT THESE E2E accessible.
3. The address exhaustion problem has mostly gone away as a result of developers switching from protocols which were NAT unfriendly to protocols which do not give a damn about how many NATs they traverse.
No v6 $deity will come even close to admitting any of these. They start any of their talks with "v4 is obsolete". They have made their careers on making v4 and v6 incompatible and pushing v6 at any possible occasion (v4/v6 DHCP is a prime example). You are more likely to make them commit sepuku with a blunt kitchen knife than admit that v6 NEEDS a major revision with half of the features obsoleted straight away.