I have a rudimentary understanding of COM and I know that its main purpose is to enable interoperability between programs written in different languages. However I dont understand why that is useful to the DirectX api. Is it because DirectX has to make calls to the windows api which are written in C whereas DirectX is written in C++?
C++ has no defined ABI, so C++ programs can't link to libraries that were compiled with a different compiler. In addition, MS's Visual Studio C++ compiler is not ABI-stable, so you don't just have to use the same compiler: you have to use the same version that the library was compiled with.
This would be a huge problem for a widely-used library like DirectX.
COM doesn't just enable interoperability between different languages, but also between different C++ compilers. In fact, I'd say this is its main use, and language compatibility is less used. By being a COM library, DirectX can avoid application developers needing to care what compiler it was compiled with. Application developers are free to use a Visual Studio version of their choice, or a completely different C++ compiler.