I think one reason is that there's no big corporation behind it, for Rust it was Mozilla and now the members of the Rust Foundation, for Go it is Google.
But I totally agree, Nim is an awesome language and definitely deserves more attention!
"I think one reason is that there's no big corporation behind it"
I agree. The popularity of some programming languages is undoubtedly buoyed by corporate sponsorship or the association with a company. This is not a bad thing, but it means other languages struggle to generate as much interest.
Also, without a generous benefactor, open source languages have to scrape funding together piecemeal from different sources. For example, both Rust and Go have had (or still have) dedicated staff writing documentation for the language. This is a luxury that other languages cannot fund or afford.
I actually posted a Ask HN question of this very topic recently: Can new programming languages attract developers without funding?"
Python did had a couple of smallish killer applications like Zope, being used as scripting language in 3D applications, and gaining adoption among Perl users after Perl 6 never ending story.
Additionally Guido has always worked in well known companies, and Microsoft and IBM have toyed with Python on their products as well.
I think that‘s a possible path for Nim to develop. Win over a trickle of python devs who‘ve learned the problems arising once you hit a certain performance ceiling / scale. This is being accentuated by hardware & cloud developments - startup time & compute efficiency becoming increasingly important (containerization/serverless driving one, I/O performance outgrowing CPU perf. driving the other). If those trends continue, Python will increasingly be ill fitted. And I‘m saying this as a Python dev.
Except that Guido went to Microsoft to work on their new JIT project, while Instagram is also pursuing their own, and there is Cython which follows the same compile to native code workflow (C based backed) as Nim.
The biggest problem in Python slowness isn't that solutions don't exist, rather the community at large tends not to embrace them as other ecosystems do.
My experience with optimizing Python code after ~10y is that after a certain point it's just not worth it and you'd be better off rewriting in a more performant runtime once you hit that. That includes experiments I did with pypy, Cython and Numba for various projects.
But I totally agree, Nim is an awesome language and definitely deserves more attention!