One problem is the paradigm of the browser and the expectations behind it. Websites are intended to load in a second, or at worse, a few seconds, so waiting any longer for an entire application would be excruciating.

But no one minds downloading a multi-megabyte or gigabyte game over Steam or console, because users are trained to expect that to take a while.

So what's needed, in my mind, is something with a user experience between the two - for games and applications to run in something more akin to a "package manager" mode than a basic browser window. Or maybe, on the web, have some alternative content to distract the user while the content downloads.

The size of the entire download could possibly be reduced by having browsers ship with some standard plugins and modules, maybe runtimes (imagine having most of the Unity framework already in the browser.)