1. Historically it is what has been used at my firm. We have a lot of code already written in MATLAB, interfaces to internal apis, external data providers etc. Everyone at the firm understands MATLAB code.
2. It really is very good for numerical work - both in terms of speed, and clarity of the code (much better than Python and R for clarity - probably on a par with Julia).
I also used KDB+/Q very heavily in a previous job. It is blazing fast in its domain (financial time series) and enables extremely rapid prototyping. The fact that it is a combined query language / programming language is very appealing for data-focused research. I wouldn't want to write a production system in it though (though I know people who have done!)
When I was studying for my PhD I wrote a lot of Fortran 77 and IDL. Essentially because my supervisor used IDL and had a lot of code written in it, and because we were using an external tool which consumed Fortran 77 files as input.