How could you justify having WinForms and ASP.NET? Silverlight and WPF tackle two very different problem domains, and their internals reflect this. You can't have a framework as large as WPF downloaded as part of the SL runtime, so SL cherry picked its behaviour
from WPF. You wouldn't want to write a full-trust desktop application in Silverlight, and you wouldn't want to write a browser based application in WPF.
Part of the problem stems from peoples insistence on using browser based applications for systems that they clearly aren't suited to. These applications tend to be characterised by the developers having to jump through hoops to cope with issues and workround
limitations of browser applications.