One could argue that it is held back by not being based on TeX (depends on how good the export is) and emacs (probably).
You can convert org-mode files into texmacs files by exporting to latex and then importing the latex into texmacs.
Emacs cannot render latex with such quality even with image previews. Trust me, I'm using Emacs.
The typesetting algorithm is based on TeX, but it is a custom rewrite with tweaks afaik.
There are interfaces to python, maxima, octave, matlab, asymptote, graphviz, sympy etc. that let you type the code into the buffer and see the graphical output of those programs, i.e. figures. Similar to a notebook. Texmacs can typeset the symbolic equations that you get from maxima, so you don't have to see the source code with code syntax but the result a rendered equation. Does emacs have a repl that returns rendered equations?
It is also possible to use the python, maxima, etc. plugins to evaluate expressions in the body of your text in a texmacs file.
You can create presentations with animations with interactive code sessions and typeset all the math in the presentation with the algorithm derived from TeX.
The main implementation uses gnu guile as an extension language, thus if you like Emacs lisp you will get along with texmacs too.
You can link texmacs files together and create a wiki structure (think org-mode links etc.) and you can export this structure into HTML. This allows you to write a website in texmacs and then export it into HTML with your custom css.
You can typesetting algorithms in the same manner as with the algorithm* packages of latex, but with realtime rendering without having to recompile or guess the layout.
Texmacs also offers Emacs like keybindings so that you can open files with C-x C-f, move the cursor with C-f, C-b, C-n, C-p and other Emacs like bindings.