The assumption that a target (or source) device has a buffer and encoding hardware capable of delta pixel encoding is would drive costs up to the point where HDMI would not be an accepted industry standard.
If you see any of popular video standards (both connectivity and formats like H.264) they avoid framebuffer and memory requirements like the plague.
Having said that, DSC exists and has been adopted by both HDMI and DisplayPort standards. But only on the highest end of capability because it costs a lot to implement on both sides.
> The assumption that a target (or source) device has a buffer
A full-frame 8K image with 32 bits per pixel takes up 128MB of RAM. An FHD one is 16 times less, 8MB. This is not a lot of memory for an appliance in 2002, when HDMI was introduced.
Having said that, DSC exists and has been adopted by both HDMI and DisplayPort standards. But only on the highest end of capability because it costs a lot to implement on both sides.