That "some reason" is that a standard DP-to-HDMI 2.1 protocol converter can't negotiate beyond HDMI 2.0 link rates without the host computer knowing about and doing FRL training on the HDMI side. Completely unrelated to any limitations related to 6144 x 3456.
As I understand it, automatic fallback/limitation to HDMI 2.0 speeds was desired by VESA in the event of using an 18gbps cable or other signal integrity issue, so ultimately they chose to require the host to be more aware of HDMI for the converter to enable HDMI 2.1 speeds rather than requiring the converter to be smart.
Yes, as a specific example, if the HDMI sink wants DSC, maximizing the quality (minimizing the compression ratio) fundamentally cannot be done without knowing the end-to-end bandwidth.
