Switches and routers have a little thing called TCAM memory, the premise behind it is that it allows you to single-cycle O(1) lookup any ips destination. Usually to replicate it you could have a 4gb*2 preallocated contiguous buffer, but that's not something that is wildly supported or used and this completely breaks down when you expand to the IPv6 range.

The problem lies in that in a lot of cases TCAM can no longer hold the entire IPv4 routing table and now if you introduce IPv6 you are expected to handle double the routes which degrades switching performance as more active routes have to be evicted and fall back to software routing.

Routes are not the only thing that take up TCAM memory: the firewall rules, internal routing, vlans, everything becomes double and TCAM memory cannot be dynamically adjusted at runtime to allocate space so what happens is that you need to sacrifice IPv4 space in TCAM permenantly even if nobody is using IPv6.

This is where it gets worse: if you have ever attempted to use IPv6 you will notice that is significantly slower than IPv4 and that is because most ISPs simply opted to use software routing for IPv6 which coupled with 4-10 hops is nearly double the latency in some cases (0.5ms to 1ms) while having throttled bandwidth to not overload the CPU.

That's why network engineers will continue to refuse to (properly) support IPv6. If I had to guess the "properly" supported IPv6 percentage is less than 10%.