Steam is merely a new distribution system for a platform Notch has already shipped on while the XBOX 360 is a totally new platform with its own required distribution system.
Wonder how well this is going to go...I bought the Orange Box for the 360 when it first came out. TF2 has not seen any of the updates that the PC has had for the past 3 years.
They have an alternative to using Steam on PCs - they can release their own binaries, or source. They don't really have such an alternative on the XBox.