Why hasn't Windows Phone taken off?

I know this is probably opening a can of worms that will denigrate to something horrible. Hopefully, the audience here is competent enough not to allow that. That said:

Why do you think Windows Phone hasn't taken off in the market in any substantial manner? The UI is - for the most part - highly regarded as original and beautiful, the app ecosystem is mostly caught up with iOS and Android. I've seen many theories ranging from people not wanting to "bring work home", to retail issues concerning vendor knowledge. 

What do you think?