Because I've written the game in JavaScript and it renders to Canvas. It also works in the browser. To go native, I'd have to rewrite everything from scratch. And then I have to maintain 3 platforms.
Maybe Unity is the solution, but right now, it's not written with Unity.
Why? App Store exposure, client-side demand for an app, offline-first – and quite frankly, because it's written for desktops in mind and the mobile experience just sucks. Wrapping it in Cordova at least gives some control over the experience, such as: You can't accidentally swipe back on the browser, full screen, no address bar etc.
Check out the Twitter PWA (https://mobile.twitter.com). Accidentally swiping back happens to me a couple times per day.