You know, this is the same exact thing that spurred on the L4D2 boycott.
Anyway, there are likely a few reasons why TF2 updates are not streamed to xbox like they are to PC users.
1) On the PC platform Valve owns and operates the content delivery service (steam.) Valve can push whatever updates they want, whenever they want, and they don't have to answer to anybody about it. On the Xbox360 platform, Microsoft owns and operates the content delivery service, so Valve can't just throw updates at xbox users, Valve has to go through the proper channels, do the paperwork, probably pay some fees, and who knows what else. For that reason, Valve is more inclined to push fewer updates to xbox than they do to PC.
2) The PC and Xbox360 work differently. Shocking, I know. On the PC, an update can be written to the hard disk, effectively writing over the old code and replacing it with the new data. On the Xbox360, you can't overwrite the original code on the disc, so all updates have to be loaded into the system RAM in addition to the other resources the game needs. If you have too many updates, you will run out of system RAM and the game won't function.
3) I couldn't think of a third reason.
Fun Fact #1: The reason Valve released titles on the xbox is because the xbox uses directx as its media API - the same API used on the PC, so it's actually very easy to make games for both systems. This is why there are so many titles shared between the 360 and PC.
Fun Fact #2: Another company did the PS3 port of the orange box, so Valve does not support that platform directly.
Fun Fact #3: Now you know what most console -> PC ports are like. We understand your pain, but aren't necessarily sympathetic.
Edit: When I say PC, I'm talking about a Windows-based computer.