same reason why you don't see boards with 2 crappy phases but seven PCIe plus six heatpipes - marketing people like smooth transitions in features from the bottom to the top of the range. The reason why you see so many mid-end variants is because transition from ~UD3 to ~UD4 level can be done in multiple ways - there's an UD3P with better PWM, for example.
Making an OC dedicated P67 board would be an extremely stupid thing to do, IMO ... unless it costs <$100 since even the ~$150 stuff has all what it takes to max out any 1155 cpu.