It's hard to give an easy answer for that.
But let's step back a little: I played around with Julia several years ago. I don't know when exactly, probably in 2015 (?). Back then I thought of it as a faster python for science. Many things I would use python for back then I could have done in Julia too. Others I couldn't though, because the eco system was still too small. And while Julia was somewhat faster for the things I tried, it wasn't amazingly faster.
My interest in the language dropped sometime after that. But still, I felt like it was a language targeted specifically to scientists.
Many things I wanted from my analysis framework however were outside that bubble I felt. I didn't simply want to write a faster "analysis script".
I do realize however, that the language has evolved a lot and I'm happy it's finding acceptance!
Fortran on the other hand, I never really considered. I suppose modern Fortran is a pretty good language though.
So why did I choose Nim then?
It gave me:
- the ability to produce standalone binaries I could just put on any data acquisition pc without a hassle (well ok, be careful about old glibcs)
- it's fast, and I felt right at home syntax wise coming from python
- the community is amazing. The first time I entered Nim's IRC channel I noticed Araq, the creator of the language, answering random people's questions! In general the community allows for a super quick feedback loop to learn the language
- it's a pretty concise language. The whole manual can be read in less than a day
- having written some Clojure, I loved the idea of a powerful macro system
- after seeing mratsim's arraymancer library  I was happy to 1) have a numpy substitute and 2) thought if one person could write such a great library in O(1 year) it must be a pretty awesome language to work with :)
- being able to trivially wrap any C code around is super helpful
- a pretty strong type system! No more annoying implicit conversions from any type to a bool, error prone implicit int <-> float conversions etc.
I probably forgot many points, but well. The truth is of course, many languages could have worked for me, but the time I spent with Nim in the beginning was just super pleasant.
Moreover, the lack of scientific libraries in Nim was more severe wrt Julia. Sure, writing C bindings in Nim is easy, but it is not a zero-effort job: you have to properly test them to check that types get converted correctly, and you have to write some documentation. Some guy is currently checking the quality of Nim libraries , and he gave very low scores for a few libraries of mine (rightly so, IMO, e.g., ) because they lack documentation.