The reason is because Valve is constantly patching Team Fortress 2 and put them out the moment they're done (as they are their own publisher), while on the Xbox 360, patches are required to go through a certification and approval process which can take a long time.
There have been dozens of patches that have been released for TF2 since the beta which added new features, changed gameplay balance, improved performance and graphics, etc.; on the Xbox 360, I believe there has only been one so far.