Because adding the preference is much more work compared to finding another keycombination.
Control+Alt+M is perfect!
Alt-GR is near M, so 1 hand required, plus Control+Shift+M opens the monitor, so it is a logic perfect choice 
Because adding the preference is much more work compared to finding another keycombination.
Control+Alt+M is perfect!
Alt-GR is near M, so 1 hand required, plus Control+Shift+M opens the monitor, so it is a logic perfect choice