Because of HDD space. I have the rMBP and I don't want to waste HDD space on Windows 8. Besides, my old PC is actually still pretty decent. I don't mind running two machines, I have my own cloud storage on my network to share things back and forth if needed.
I understand. Also, it's not a big deal, but it bothers me that you referred to it as HDD space when referring to your rMBP, which doesn't have an HDD.