Only speculation, but it seems like the Express is a smaller router (designed to plug right into the wall), and would make sense to be plugged in behind your stereo or whatever - more like an add-on product.
The Extreme and Time Capsule are designed to be your primary router, and might not be located near your stereo, etc.
Not to say that you couldn't use an express as your main router, or put your Time Capsule/Extreme in your stereo cabinet, but it just seems like they aren't designed to be used that way...
The Apple TV supports AirTunes now, which makes alot of sense, since that woudl definately be close to your stereo.
Then again, I have hear people asking why they can't use their Apple TV as a router also...