The question shouldn't be "Why not?" -- it's "Why?". If you are running a browser, you are already using an operating system. Why would you run another one?
Running editors (or whole operating systems, for that matter) in a browser seems like a backwards way of solving this. If you really need to ship a whole development environment including the editor, and not just the config files, project files or syntax highlighting plugins, you might as well use a virtual machine. If a programmer has no idea of how to set their editor of choice up for what they're developing, they won't be building a decent development environment in Javascript.
My point is that the web browser IS a virtual machine! It also happens to be the VM with the largest install base. It is a lot easier to ship something that runs on this VM than to convince people to download yet another VM.
And this isn't about developers knowing how to set up their editors. It is more about DSLs. It is more about the fact that there is more to coding than just writing source. Hell, it is about trying to come up with a way that we aren't totally beholden to source! It's just data, and frankly, not the most important data around software!
Sometime other paradigms, and I don't just mean other languages, but things like spreadsheets, graphs, and visual languages can be great DSLs that properly model and convey information MUCH better than source... but you can't really embed those in eclipse or vim, can you?
Think about if you released a project on GitHub and it didn't have tests, documentation, or example use. Would anyone use it? What if it was competing with other projects that DID have tests, docs, and examples? What is that code with tests was slightly worse? Which project are other developers going to want to interact with and use?
We live in a world were our programs, our source, and our peers live in a distributed ecosystem and as far as I can tell our tools and operating systems are starting to get in the way.
Have you seen how awfully messy web development becomes when you try to bootstrap from a filesystem? Things like JSFiddle are MUCH more elegant, but are, how shall I put it... missing some key tools for a new ecosystem of distributed computing. ;)
Running editors (or whole operating systems, for that matter) in a browser seems like a backwards way of solving this. If you really need to ship a whole development environment including the editor, and not just the config files, project files or syntax highlighting plugins, you might as well use a virtual machine. If a programmer has no idea of how to set their editor of choice up for what they're developing, they won't be building a decent development environment in Javascript.