Maybe it's related to the reason why you can't use the 5k iMac as an external monitor. I can't remember where I found the sources, but the problem was that they essentially needed to video cards to drive the thing and making sure that both sides of the output looked identical was tricky and not something that an external source would be able to do.

IIRC it was an issue with the video connection. Nothing at the time could provide enough bandwidth to support 5K, so Apple had to cobble something together. With USB-C and TB3, that's no longer an issue.