Mainly because number crunching code tends to be very long-lived (hence why FORTRAN is still in use).

Not only that. Fortran is very good for writing number-crunching code. Modern Fortran is a pretty ergonomic language, it gives you a really easy way to auto-parallelize things in many ways, and new Fortran code is being produce unironically. Of course it normally uses the treasure trove of existing numerical Fortran code. (Source: a friend who worked at CERN.)