A lot of the early, absolute advantages of Fortran (numerical precision, non-aliasing) has been adopted by C.

However what still makes it a great choice is that it still a relatively simple language compared to C, and maps mathematical formulas to code very well. Inelegant code will still run reasonably fast (which is not the true with C) - so all of these points make it actually a good language when the program is being written by domain experts (i.e., scientists) rather than professional programmers or software engineers.

May I ask: why did you write that library in Fortran? That is, what was your answer to your question at the time you wrote it?

I have heard that Fortran is very very good at optimizing array processing due to lack of aliases. That may be one answer today. There’s also value in continuity: in using a language for your task because it can build on what others have built. That applies to any established language, of course… but I could see it reasonably applied if someone wrote some kind of numerical processing library today. They’d write it in Fortran and call it from Python!

I mean, I think it's best in class in its niche: writing numerical algorithms that compile to highly optimized machine code, while being reasonably readable and intuitive (at least from F90 on). It's arguably still the easiest way to write compute-intensive numerical extensions for python (well, if it's single-threaded, and if numpy can't do it, and if you're not using GPUs ...).