http://www.geekwire.com/2012/mobile-game-developer-html5-futureWhile the article pertains more to the mobile game realm (my day job), I think it applies to your question too.
As for large files, well... users still need to download the HTML pages, and all the related content, which takes time and disk space in a cache. Or they can save it locally for an increase in speed, but they're still taking up all that disk space.
Another issue is distribution model. HTML 5 is inherently open. If you have a bunch of HTML, JavaScript and images, that's pretty easy to pass around for free. It would be very difficult to monetize that.
Then there's the issue of speed and simplicity. If you're writing a game in HTML 5, you're writing in JavaScript, a parsed or JIT compiled language at best. The simplicity of writing a game in JavaScript is essentialy useless, as you can get the same simplicity in any sanely-designed language, which can be natively compiled to whatever architecture you're running on, which allows for more FPS, the almighty benchmark.
The one thing HTML 5 supplies is a true write-once-run-anywhere scheme. But you can get the same benefits once you have a framework written on each platform you plan to develop on. And in general, C++ is "portable" across platforms anyway.