I believe the reason, IIRC, is it's actually doing two DP streams because one isn't enough bandwidth for the full resolution, which you can't do in alt mode where the PC is literally using the pins of the connection as though it were a single DP cable (although this may be more viable with newer DP specs these days, not sure).

The DP altmode didn’t provide enough bandwidth, which is why the best experience is over the Thunderbolt alternate mode.

edit: you can in fact get HBR3 working, what's not working at full rate with DP instead of thunderbolt is the USB3 hub on the display.