That's because the browser had to do things the sneaky way.
Everybody wants to control the app platform. Every time a cross platform solutions appears everyone tries to shut it down. See: Java, Flash, etc.
The only one that platform owners couldn't really shut down (though they are getting better at it), is the web. It was considered too "dumb" to shut down and too useful to completely avoid. And bit by bit, just like the boiling frog story, people added interactivity features until we arrived where we are now. A bloated, perhaps crippled cross platform solution, but the best we have now for running things on things from embedded, mobile, desktop, etc., etc.
