http://www.infoq.com/news/2016/03/msuwp
Universal Apps for Windows 10
http://www.slideshare.net/shahedC3000/universal-apps-for-windows-10
Windows 10 introduces several new APIs and concepts (such as API contracts) that are not available on Windows 8.1. An app that relies on such new APIs and concepts cannot run on Windows 8.1. Additionally the app-model and a lot of packaging machinery for Windows 10 Universal apps is very different from Windows 8.1. Apps built with such new information cannot be successfully understood by Windows 8.1 for deployment. Also, Store will not make the Windows 10 apps available for download on Windows 8.1