Because like it or not, it is a right handed world out there and as the minority it behooves us to adapt and overcome to the world around us.
There is no need to "adapt" to a right handed world when there are left handed options available.
Trying to sell a left handed gun to only 15% of the population is problematic and what happens when you need to borrow or for whatever reason use a right handed firearm? Or if in an emergency your right handed compatriots need to use your firearm?
I have never purchased anything for myself thinking that someone might need to borrow it, and I should by it in there "hand" instead of mine.
Yes, a left hander may have to use a right handed gun at times. Does not mean he has to purchase his own wrong handed gun.
In 30 years of shooting sports I have only owned one purpose built left handed firearm and that is a flintlock, for obvious reasons. Otherwise I simply cannot see the purpose of left handed firearms. I have buddies that are also left handed. They sure converted to left handed guns once they shot mine. You should try it.