[cadetzero]

Because if you're creating a 2d game, it can be rendered reasonably fast in your browser.

Full 3d is a different problem.

However, a game like Monaco, with super simple controls but really interesting game play, could absolutely be built in the browser. This gives an easier distribution medium, more control over who is using the game, and if you're already a webdev it fits in nicely with your pre-existing skill set.

The bar is significantly lowered for a lot of weekend warrior game makers if you're going the 2d route.