It's really not. It is simply very expensive to build up experimental validation of simulation code rewritten in a new language, and much more expensive than the aesthetic value given by programmers who want new things. There's nothing magic about the old things, there's just an existing ecosystem which works which would involve a whole lot of effort to recreate and most people think they have better things to do.
More likely, a rewrite doesn't look good on a quarterly spreadsheet and "shockingly" code designed by Johnny Two-Shoes in the 60s that uses COOMON blocks both for internal storage as well as an API is incredibly difficult to test and thus incrementally modify. Bonus points for next to no documentation.
College software engineering textbooks that explain why global memory is generally a terrible design choice were written off the lessons learned from legacy Fortran.
