well to undergo fermentation, a micro organism (yeast) needs to be used.
yeast ferments sugars into ethanol to create energy for its metabolism, and thats why fermentation reaction happens.
however in an aerobic environment, the yeast will RESPIRE rather than FERMENT the sugars to make energy, and so fermentation DOESNT happen, respiration happens instead
however when there is NO oxygen (anaerobic) then respiration is impossible, and the yeast metabolises by fermenting the sugars into ethanol
thats why it needs to be in anaerobic conditions
does that make a little more sense???