I am not personally involved in the development of a large-scale-library but I guess one factor might be that C is kind of the lingua franca of programming with respect to the fact that a lot of programming languages provide standardized interfaces to the C programming language.
I know that is is also possible to interface Fortran code like it is done for R or SciPy. But as far as I understand, this has to be done either in a non-standardized way or by using C interoperability by both Fortran and the other programming language.