There was widespread belief for years that the project was dead. I'm sure that didn't help.
The interface was somewhat clumsy in the early years (around 2005 or so). It wasn't responsive and a lot of things were unintuitive.
Some things simply didn't work. There was a concern that if something broke, it might never be fixed.
Keep in mind that my usage ended about 15 years ago. Maybe it's better now, but LyX was a good alternative, and I've never had a reason to move. It can be hard to overcome a slow start.
Unintuitive interface, does not mean bad. Means just TeXmacs is different from other software (like LyX) for example. But the interface is well thought and carefully implemented. It requires some time to get acquainted but then you can do many things are not possible with other software (like shown here: https://www.youtube.com/playlist?list=PLjXdYclFpynD0ufg_pp-5...) And is not dead. :) We are a small number of developers but the software is used every day to write academic papers with a lot of math from a very affectionate user base. If LyX works well for you no problem. But you should give it yourself some time before making a judgement. It is surely very different from LyX.
Oh, I definitely don't want to discourage anyone from using TeXmacs. It is a great piece of software. I was only explaining my thoughts on why it's not more popular. I think a lot of people would like it if they had ever heard of it.
The interface was somewhat clumsy in the early years (around 2005 or so). It wasn't responsive and a lot of things were unintuitive.
Some things simply didn't work. There was a concern that if something broke, it might never be fixed.
Keep in mind that my usage ended about 15 years ago. Maybe it's better now, but LyX was a good alternative, and I've never had a reason to move. It can be hard to overcome a slow start.