Ok, I think I get it.
You're simply not the intended target of the 12".
People who generally buy the 12" are looking for the smallest/lightest that will run OS X. I think a lot of us also have multiple computers (I have a 15", and a 12.9" iPad Pro).
Here's the 13" vs 12":
View attachment 628513
If you look at that and say "Its not much of a difference, I'll take the 13.." - then you're not the intended target..the person looking for the lightest/smallest.
With me, I take a backpack filled with supplies to work. The 12" fits in the tablet section of my backpack, and allows me something to run xcode/logic pro x.
I had the 13" before, and the 12" is a huge improvement for me, in terms of portability.
Hell, it's even easier taking with me than my iPad Pro...and I get to use Logic Pro X and Xcode!!