They have to update their notebooks with Thunderbolt 3 before they can release 5K Thunderbolt Display. Thunderbolt 2 doesn't have enough bandwidth to push the pixels.

It's a technical limitation, not an oversight. No current Mac can output that much data over a single port.

With the amount of custom chips they needed for the display, it wouldn't surprise me if there are issues getting it to work as an external display that they haven't solved yet.

I think they might be waiting for a bigger refresh of the Mac Mini to power it