Javascript has big performance limits which don't allow Maps to do everything that Earth does. The 3D engine in Earth does a whole lot of magic to get the visual quality that it has. In Javascript, you have to trade off everything to minimize your function call overhead into WebGL and try to pre-generate as much static content as possible. Earth has a much quicker access path to GL, which allows it to render more and higher quality visuals.

Satellite mode without terrain can look fine in JS/WebGL. Once you start tilting the view and seeing terrain and buildings, the JS performance will be horrible pretty fast, and adaptive LOD streaming is hard due to the function call overhead.

I worked on the Google Earth desktop app for a few years, so this isn't idle speculation.

This new NaCL version seems to be nowhere near feature parity with the old desktop client, sadly. I hope it catches up, since I loved the historical imagery, for instance. The good news is that the desktop app still works if you want to see that.