I posed this question in a post over on itjoblog. There are several reasons why Silverlight struggles to get designer attention, including:
1. Designers are pragmatic and target the runtime that is already deployed most broadly, ie. Flash.
2. Flash is already good enough so why bother?
3. The tools: Adobe’s designer tools are a de facto standard, target Flash, and run on the Mac.
Developer is another matter. The cross-platform .NET runtime is Silverlight’s big advantage; and this time the tools tip the balance towards Microsoft (Visual Studio) – not for everyone, but for the substantial Microsoft platform community. That’s going to be further reinforced by Visual Studio 2010 which gets full visual designer support, plus of course Silverlight 3.0.