The problem with mobile HTML5 is that the frameworks aren't nearly as sophisticated as their native counterparts. There's nothing that even touches UIKit on the web. Things as simple as drawing a UINavigationBar-style gradient prove to be unbelievably challenging. On a technical level, it's completely doable, but it requires a lot of time, effort, and designer time. Multiply this out by all the little niceties like animations and embossing all of the buttons and you've got a big job ahead of you.

All the technologies are in place to have a native-quality HTML5 app -- we have canvas to draw pixels however you want. We have WebGL to do any 3D rendering. We have blazing fast JS engines. We have hooks for things like location and camera roll access. The problem is that it's really hard to justify the investment of building out a UIKit-quality framework for the web. Apple and Google can spend hundreds of millions of dollars on their frameworks because it means they'll sell more devices/services. But it's really hard for someone to justify doing so for the web, where there's no ROI for your platform.