Both:

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.

Javascript is an expressive language and with V8 that makes it a compiled language (with good performance) creates an impressive combination.

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!

“Java is to JavaScript what Car is to Carpet.” - Chris Heilmann