Well, the reason is not so weird. Generally speaking, the more powerful the tool, the higher the bar. More time and effort is required to learn it and become proficient with it. When it comes to the very powerful tools, few will have the aptitude or be prepared to put in the effort to learn them.

For those who don't, less powerful tools take their place and proliferate.

As you say, emacs checks all the boxes but the majority is not prepared to learn it and prefers to program throught their browser.