There's also the overhead of supporting/integrating multiple developer/build/test environments to add to the things Abigail has already mentioned.

Depending on the people, project and company the overheads of supporting a heterogeneous environment are sometimes not worth the effort.

One of the larger development mistakes I made was selecting Perl as part of a web-based application when the rest of the development team was using C++. While Perl was definitely the better choice technically for the particular problem I was solving, it was definitely not the right choice in the wider context of the organisation as a whole since it added an extra layer of overhead. It eventually got re-implemented in C++ and this was the right decision to make IMO.