The same reason you don’t use the PIT knob to set the pitch in this state, or the LEN, if you use the matrix.
It’s about the circle becoming a dedicated area for quickly changing the values of tracks and steps… You see the pitch on the inner ring, and the velocity on the outer ring, and (step) length in the matrix in EDIT mode.
At least from the use paradigm I see, if the "value" of the parameter is being displaying in the numeric section of the circle, the master knob should modify the parameter being represented by that number. Just like setting the tempo.
How does everybody else feel about it?