The biggest reason is probably that you don't need the user to explicitly download and install your app. Also, a web app works on all the major OSes with a sing

The biggest reason is probably that you don't need the user to explicitly download and install your app. Also, a web app works on all the major OSes with a single GUI environment. And then of course the fact that webapps run in a more secure sandbox. A standard cross API and sandbox could potentially solve the second two. But I'm not sure how to solve the first outside of a browser.