I think that JS is way less verbose than Java/C#/C++ which are very popular backend programming languages.
I don’t think that JS is more expressive than Python, but here comes the speed/performance into effect:
Using python or any other interpreted language as BE is very slow and hence limiting.
Add to that, per Guy’s comment above, the great community and libraries/micro-libraries and the deal is sealed :)
Important to mention: all this is just my opinion (based on my knowledge and experience) — so feel free to counter if you think otherwise!