The word 'why' should be removed from the title. Usage statistics are fine, and TPM admits to not being a representative demographic of the Internet at large, but the blog entry doesn't attempt to answer the question of 'why' at all.
tl;dr: In November only 56.81% of visitors to talkingpointsmemo.com were using Windows. Windows is losing ground to OSX but mostly to mobile.
It would be interesting to compare this to other sites. I'm sure there's a demographic that's influencing things on TPM.