Born at the end of the 50s, during the 60s and the 70s it became the most popular choice for the development of numerically intensive code. An enormous body of knowledge has been embedded in Fortran codes. C is a more recent language. It was developed in the 70s and provides the foundation for the Unix operating system. The popularity of Unix is a direct consequence of the fact that it is largely written in C, and therefore easily portable between different brands of computers.
Languages evolve much more slowly than computer architectures.
However, advances in computer systems often create a need for new features in a language. For this reason, even a stable and well-proven language such as Fortran undergoes periodic revisions in order to keep it up to date. At present, there is an immense amount of scientific codes written in Fortran 77. Many of them are of public domain, and among them there are libraries of subroutines for solving a large variety of problems. Both Fortran 90 and C are perfectly valid choices for a computationally intensive application.