Matlab is very, very heavily used as is R and Excel.
The problem with mathematica for hardcore derivatives pricing and algo trading is that it simply can't handle the volume and have the speed that you need for that sort of work. Also for that sort of work, you want to be able to tweak the internal models and implementations, which means C++.
One other thing is that standard math engines often do not have the features and functions you need, and so most investment banks have an internal math engines that in some situations do what mathematica can do. Wolfram research is a very small company, and the major banks have more programmers and money than is available to Wolfram is often if you need something similar to, but not identical to Mathematica, it's often much cheaper and faster to write your own version of Mathematica, and if you rewrite Mathematica, large parts will likely be in C++.