Because iTunes is used for iPhone, iPad and iPod Touch which are all different kinds of devices. Not to mention that iPhone, iPad and iPod Touch have various models. It would mean that iTunes would need to do the same thing the App Store is doing. Simply put, it would require you to run your own App Store proxy.The question you should be asking is, why doesn't iTunes look at the current version of the app it has and then only download a delta update to the latest version just like iOS devices have been doing for a long time?
He said that Apple has deliberately disabled transfer of Apps (Apps syncing) from iOS 9 Beta 5 onwards till iOS 9.0.1 to stop spreading of malware which was propagating through the app store. Something to do with malware infected version of XCode uploaded to Chinese servers few months back.