This and nine more CoffeeScript derivates* from people who rather like to fragment a landscape with marginal improvements instead of supporting the leading pack are the reasons that CoffeScript hasn't reach critical mass and will fail as a standard.
CoffeeScript hasn't attained critical mass? It's the 10th most popular language on github[1], over a decade younger than all the other contenders, and growing quickly.
Even if CoffeeScript was as niche as you argue, I'm not convinced that that would have any bearing on its success. CoffeeScript doesn't need a wide community to achieve success because it compiles to and is completely interoperable with JavaScript, the most popular programming language.
Regarding the front end side the CoffeeScript adoption is high and mainly driven by Rails but if you look at the back end it's different: CoffeeScript could not established with Node (most npm packages are in plain JS) and mixing Coffee and JS on the backend does not really work out (it's possible though).
Even if CoffeeScript was as niche as you argue, I'm not convinced that that would have any bearing on its success. CoffeeScript doesn't need a wide community to achieve success because it compiles to and is completely interoperable with JavaScript, the most popular programming language.
[1]: https://github.com/languages