As many of the comments have already mentioned, there are several different reasons people might recommend the use of Fortran over Matlab. One of the most straightforward answers is that a lot of legacy (read: validated) code is written in Fortran, and depending on your job function, learning to use Fortran might make you more productive - for instance, if you have to use, maintain, and extend these tools.
Another reason (also mentioned in the comments) is, basically, performance. Well-written Fortran code is about as performant as you're going to get, and Fortran has the added benefit of being a farily straightforward language in which to write scientific code. Solving differential equations is computationally intensive, and since that is a core task of most astrodynamics code (orbit propagation), it makes sense to stick with a language that can give you good performance. Modern astrodynamics work often requires solving these differential equations many times over (e.g. for Monte Carlo simulations or optimization tasks), making performance even more critical. Of course, one could write performance-critical code in Fortran or C and link to the libraries in Matlab, but often it's easier to be consistent and stay in a single environment, particularly for students with limited time to learn multiple languages (although this is mostly a matter of preference).
Finally, Matlab has several design flaws (which I won't elaborate on here) that make it a poor choice for implementing a large, complex codebase (although, of course, many large codebases have been written in Matlab). Astrodynamics code in particular has a tendency to get complicated quickly for even basic tasks - for example, you may want to "just" propagate an orbit, so you need a flexible ODE solver, but you also need to simulate various perturbations, handle reference frame transformations and different time systems, etc. A flexible astrodynamics codebase can be sprawling, and I know from experience that it gets extremely messy in Matlab. Fortran is well-suited to this type of code. It's worth noting that it's not required to use Fortran for performant, well-structured codebases - the Julia language is one example of a "high-level" language that does not have the deficiencies of Matlab.