Netflix chose MSFT's Silverlight for handling PC streaming. Lack of silverlight on Linux is the problem.
Step 2 would be to ask why Silverlight is so important to them.
Step 3 would be to ask what does Step 2 have to do with anything?
The reasons for their choice have everything to do with why they made the choice.
Silverlight 2 is supported on Linux by Moonlight as of late 2009. However, the implementation is lacking support for Microsoft's PlayReady DRM.
Netflix is available on Roku, GoogleTV, and the Boxee Box which all run Linux kernels. Lack of DRM is the problem. I believe the interface is authored on top of Qt and DRM is handled by special hardware in these devices.