It is because a DisplayPort HBR2 transmission is limited such that 170 Hz and 10 bpc are not possible at the same time at 1440p.
It is a straightforward bandwidth limitation and is not related to how 10 bpc color depth is implemented (FRC or not). Since the monitor advertises "DP 1.4" hopefully this means it supports HBR3 speed, so this would not apply. That is what the discussion above was about.
Panels are addressed in a matrix format where only 1 row of pixels is actually connected to the controller at any given time. The controller scans through every row in order over the course of 1 refresh cycle. It is not possible to go back and change the color of a pixel twice during one refresh cycle as this would simply be the same has having a twice as high refresh rate, and this capability would not be wasted on simply implementing FRC.
FRC operates by changing the color on alternating refresh cycles, not within a single cycle.
My suspicion is that the monitor only supports HBR2 speed, and when ViewSonic says it has "DP 1.4" they just mean it supports HDR or something else. I should note the user manual states only "DP 1.2".