One difficulty with Python for teaching is that it is rather complex to get students set up and running with python+ipython+numpy+scipy+a compiler+... There are so many combinations, every student has a different version of a different operating system with different other crap installed. Matlab is matlab is matlab. Sage seems to come reasonably close as an alternative, though in practice there are often still issues with setup. (Maybe minor enough that the other advantages of python outweigh...)

The other advantage of matlab is that it is more friendly to non-programmers. There are a lot of people that get lots of real things done in matlab due to all the semi-idiotic design features that annoy serious programmers. (Monolithic namespace, "everything is a matrix", one function per file, etc.) The huge advantage of "copy on write" is that you don't even have to really understand the concept of "different copies of the same data". That's a big hurdle coming to python-- a hurdle you might not want to get over if you don't care about programming, and just want to solve your problem.