Support of .NET framework - if you are already .NET developer it is easy to start programming on Silverlight.
Support of Managed code - you can write programming in your favorite language which .NET CLR supports like C#,VB.NET, Dynamic languages(IronPython, IronRuby).
Better development tools -Visual Studio 2010, Expression Blend.
Large community- available lot of learning resource as compare to Flash technology.
Integration with Enterprise based technologies like WPF, LINQ etc...