Partially historical inertia, partially the fact that commercial Fortran compilers have had decades to optimize numerical code to within an inch of its life. A

Partially historical inertia, partially the fact that commercial Fortran compilers have had decades to optimize numerical code to within an inch of its life. Also, for at least the older versions of it, its a pretty small language. If you have a grad student who's already familiar with programming, they can probably teach themselves Fortran and start being able to make useful changes to your research code in a week or two. Compare, say, C++ where just learning the language enough to be useful could take most of a semester.