I'd argue it's to keep developers locked in to Objective-C and iOS. Facebook tried going the HTML5 route in the native iOS app, but everything was too slow (it was comical how much faster facebook.com was in Safari vs running the app) until they ultimately switched back to Objective-C.
It's ironic that Objective-C was an albatross on the neck of Mac OS X, a platform with small marketshare and an uncommon application framework. Now, Objective-C is a key advantage for Apple's vendor lock-in keeping iOS developers from (easily) porting to Android.
