In March 2015 NBC News refreshed their Today app on iOS and Android with some significant new features, including off-line viewing and a split-screen mode which let you watch the news videos while reading other stories.
It seems at the same type NBC realized they had a Windows Phone app, which was last updated in April 2014, and in the absence of any plans to bring it up to date has chosen to unpublished it instead.
Not that the app had been working well before this, with users noting some categories in the app had news which were more than 8 months old.
For these entertainment-type apps a universal app makes a lot of sense, and can offer features beyond a website, such as the multi-tasking feature which the iOS and Android app offers. Hopefully NBC will realize the benefit of such an app in the near future.