Thats a very good question. It is probably something to do with the hardware components of that keyboard.
- Apple TV (2nd generation or later)
- Apple TV Software 5.2 or later
- Apple Wireless Keyboard *
Third-party Bluetooth keyboards that use the Apple keyboard layout may also be compatible. Check with the keyboard manufacturer for additional information.
* The original Apple Wireless Keyboard (2003), that features a white case and uses four AA batteries, is not compatible.