How come some Netflix shows aren't on the Wii but are on other devices?

I have an Xbox 360 connected to my living room tv, while my Wii is connected to my bedroom tv. For some reason many tv shows that are available to stream instantly are on the Xbox 360 and PC but not on the Wii. A few of these shows are The Office, My Name is Earl, and Firefly, among others. I've read the Wii can't show HD programming, plus my living room tv is widescreen while my bedroom tv is standard. Any ideas?