This is because Skype no longer supports Windows 10 Mobile/Windows Phone. Yes, the new app is still a Universal Windows Platform (UWP) service, but the framework has changed. Mobile support on Windows has been cut, so along with it an important feature. Users of Skype (UWP) on Windows 10 Mobile could select the app to be their default messaging service. The main benefit of doing this would be to allow Skype to send and receive text messages. As the feature was always hit and miss, it seems Microsoft has decided to kill it off. Overall, the new Skype app based on React Native is a better service than the UWP app, which was often buggy and frustrating. We expect Microsoft to incrementally update the new app to bring it closer to the full functionality of desktop.
React Native
As Microsoft described when it announced React Native support, the Facebook framework is compatible with UWP. This means the app still supported Windows mainstays such as having a Live Tile. Furthermore, the app will still function across form factors and platforms. In fact, with React Native under the hood, Skype may become more universal across devices and services. When the new framework was announced last month, many observers were concerned Microsoft was ditching UWP. Microsoft and Facebook have previously said that React Native and UWP can work together. Using React allows developers to create applications for multiple platforms and easily move them to Windows with little recoding.