The thing about scientific computing is that scientists like to write MPI and Fortran. They just love that shit. And they are traditionally really resistant to any new programming model. So when you tell them they need to start using XYZ instead of MPI so their programs can actually complete at exascale *before* hardware failure, they get unhappy and instead implement things like checkpoint/restore that takes 70% of the runtime. Source: I work in HPC.
Changing from FORTRAN would require us to actually try to comprehend decades worth of scientific data stored in FORTRAN data files. Ain't no one got time fo' that.