I have to admit though: Netflix's (Silverlight) player is remarkably stable compared to anything I've ever tried in Flash.
Silverlight can enforce protected streams to the output devices. I couldn't play netflix streaming movies on my old monitor since it didn't support HDCP.
They use it for the DRM. In their case it's a necessary evil.
Some sort of deal with Microsoft I would assume.
.NET.
Almost everybody uses some version of Windows or other and so .NET is there more or less.