The most compelling reason to use Objective-C for iOS development is to make use of the vast array of device-native APIs provided by Apple, most of which are not available to webapps as of now. While you can use PhoneGap/Cordova to wrap your webapp in a native container, and access many native APIs, the process is convulated to say the least. Besides, with Apple releasing new API's(1500 new APIs for iOS 7 [1]) with every upgrade, the likes of PhoneGap cant really keep up with natively available functionality. Performance is another big reason which comes to my mind.

[1] http://www.macrumors.com/2013/06/12/upcoming-ios-7-apis-gami...