Because the whole point of the editor is the web stack.

The fact that it's built on web technologies, and web developers can make plugins easily and quickly and can hack on the core code is the entire reason for it's existence, and why it's so popular.

If you rewrite it in another stack, you basically just created an entirely new editor which isn't compatible in just about every way with atom.

Great comparison. In both cases choosing the language that helps you launch quickly is almost always the right decision. You can always scale later. Better to find out if your app is even going to be a success, first.