Because in designing the iPhone 5 Apple needed to use an Updated version of iTunes in order to offer all the new features that the iPhone 5 offers. This is discussed in a Support Article on the Mac's OS X Requirements for syncing with the iPhone 5. Apple has always had to update its Apps and OS's in order to offer new features. You just haven't Upgraded your Mac and its version of OS X in a long time. Now would be a good time.
Hope this helps