I'm no medic, but it is the partial pressure of oxygen that is important to sustaining life. Think about it this way:
The O2 you breathe through a mask is at the same pressure as the cabin - if it were at any higher pressure it would inflate your lungs like a balloon - and with the same consequences if the over pressure were too great. Divers have to be very careful of this when ascending.
As the cabin pressure reduces, so does the pressure of the O2 from the mask. Less pressure means that you are inhaling less oxygen per lungful. Also less pressure means that the chemical processes that absorb O2 into the blood work less well (in fact at some critical pressure they actually reverse and you start to lose O2). As the pressure reduces there will come a point that the human physiology cannot oxygenate the blood sufficiently quickly and cogent brain function is the first to go.
Imaging breathing pure O2 at a pressure only fractionally above zero and its intuitive that there is very little actual O2 being inhaled.
Hope this helps.