They haven't done this because the Servlet 3.1 spec has not been released yet. Play 2 supports some features that cannot be supported with Servlet containers <3.1 (probably stuff like web sockets). However, there is a 3rd party plugin that provides this functionality (as long as you avoid the unsupported features): https://github.com/dlecan/play2-war-plugin
Whether it's labelled Silverlight, WPF or something else is secondary: Xaml and .NET is will be a first class development environment on Windows 8, as was shown today.