WinRT XAML does not support hehaviors out of the box. As to why this is, one can only speculate. This library tries to fill the gap for people who really need behaviors to get cracking in XAML for Windows 8 apps.
Install-Package WinRtBehaviors -Version 1.0.0
dotnet add package WinRtBehaviors --version 1.0.0
This package has no dependencies.