A large chunk of IT people don't understand it. So they don't demand it from their vendors. The vendors see the lack of demand so they spend fewer resources on it. When someone wants to implement IPv6, they see the limited vendor support and decide against it.

The last three companies I've worked at have all been IPv4 only. I set up IPv6 in our lab, but neither the SCADA software nor the PLCs we use support it at all, and none of my coworkers can troubleshoot it if problems pop up.

I've got a buddy that's a technical manager at an ISP and he'll straight up tell you they don't do IPv6 because he and his people don't know it.