Microsoft has a mobile problem. Despite being one of the first companies to see that the future was in mobile computing (it released the mobile operating system Win CE back in 1996, and Bill Gates predicted the modern smartphone back in 1999), Microsoft has never had much luck getting its mobile gadgets mainstream.
Its efforts improved enormously with Windows Phone 7 and 8, and Windows 10 Mobile is actually very good, with strong hardware. Unfortunately, it's dealing with the entrenched Apple and Android, and it has one huge weakness: apps.
Windows Phone and Windows Mobile aren't compatible with existing apps on the market, which means developers have to create new ones. That leads to a Catch-22 where people aren't going to buy a smartphone without apps, and developers aren't going to make apps for a platform with no users.
Hence, Microsoft still hasn't had much success, and its app library has been growing only very slowly. However, some recent developments are changing that.