Basically, the problem is that the Web browser doesn't give enough control to developers to be used as an application platform. Simple things like key bindings are impossible for example (because there are some key bindings that the browser will not let you modify).

Another problem is integration with other desktop applications and system utilities. The browser intentionally removes and restricts this functionality because the browser's model of the users is that users are incompetent bumbling idiots who must be protected from bad choices at the cost of being able to perform useful actions if necessary. Emacs has some safeguards of that sort, but they are intentionally made easy to disable (eg. loading directory-local code).

Emacs itself serves as a communication interface for other programs and uses operating system interfaces to the same effect. It's trivial to run a program from Emacs and feed its output to another program, while displaying the result in Emacs. In the browser, you'd have to arrange for a custom protocol, a server that listens to requests from that protocol, and write wrappers around every utility you want to run so that they can wrap their outputs into messages of that protocol... essentially re-implementing IPC system abilities in browser (but a lot less efficiently).