To be honest I think we need to stop thinking that "cloud development" needs to be in a browser. People like to do all sorts of things with their IDE that I certainly hope browsers never let pages do, like intercepting certain command keys, controlling the layout of multiple windows interacting with and knowing more about the user's display dimensions and multi-monitor, etc.

Not to mention things like vertical-selection, or multiple cursor modes for column editing, which can't be done at all in any of the current web-based editors which AFAIK are all based on contentEditable rather than a custom renderer like the original Bespin (I think, that thing changed codewords a lot).

Now some of these can be addressed with Electron or similar, but there's a lot of things that are nice to do in a code editor that just aren't compatible with the DOM itself, and need a deeper customisation of the rendering and input management.

TL/DR - it's mostly UX :D