If you ask me, the number one reason is that Android and iOS have UI libraries that are specifically meant for mobile devices. Yes, you can design a web app for a mobile phone, but there is nothing that guides you or gives you any help in that direction. For example, navigation between screens. On iOS, you have a nav bar at the top of the screen, and when you navigate to a new screen it slides over the current one, a…
Of course everyone hated it because performance sucked and interactions weren't smooth and reliable.