HTML5 is great because its versatileits not specific to a single use case. More important, HTML5 is ubiquitous. Its on your PC, your phone, your slate devicefor all I know, it could even be on your kitchen appliances.
Take these two properties of HTML5versatility and ubiquityand it should become pretty clear why so many developers are inspired by it. And, as the proverb goes, When developers are inspired, they usually write games. (Okay, maybe I just made that up.)