For one, CommonJS doesn't work well in the browser.
Browserify and Webpack bundling seems to work pretty well.
I agree that getting ES6 modules in the browser will be nice, in the end, I see it working in practice a lot like it's implemented in Babel, to work with CJS modules.
To be fair, most people should not be owning/driving cars. Walking is completely reasonable in dense cities, and yet they persist.