According to this site Windows platforms make up 92% of the market and OS X platforms about 7%. Granted there may be a higher percentage of photography users who are on Mac platforms, but I doubt more than 2x (i.e. ~14% market share).
With such a small percentage of the photography user market it's surprising Apple has done so well with Aperture. Perhaps Apple looks at it as a vehicle to entice more people to buy Mac hardware and doesn't mind selling it at a loss. Obviously Adobe doesn't have that option –Just a thought.