My outside suspicion is that they are (were who knows now with the merger) waiting for Wayland to get to point they felt comfortable supporting it, that or they were waiting for the container ecosystem to stabilize enough that they felt comfortable backing their horse.

Although with the merger I'd assume it's going to be a bit longer regardless.

Holding back a server OS (face it, that's what RHEL is) for good Wayland support makes no sense. But figuring out their Docker/CoreOS/Atomic/OpenShift strategy before RHEL 8 would make sense.