Unity still doesn't have great support for making pure 2d games of the Mario/Metroid/Castlevania variety and is awfully heavyweight compared to an HTML5 canvas.
That, and the accessibility of the API vs. having to download and work in an IDE that you'll eventually have to pay money for mean more JS games.
Phaser.io[1] is a fairly comprehensive JS engine that is totally free. It's used to make professional games that run across a variety of devices.
They added 2d support in 4.3 a couple of years ago, but it was still really clunky then. Like, you still had to texture wrap 3d objects behind the scenes and it made 2d GUIs painful to use with no native pixel measurements.
That, and the accessibility of the API vs. having to download and work in an IDE that you'll eventually have to pay money for mean more JS games.
Phaser.io[1] is a fairly comprehensive JS engine that is totally free. It's used to make professional games that run across a variety of devices.
[1]: http://phaser.io/