This model is a part of a more comprehensive model which has to be communicated with another Python library to get the stimulus (see the link above). It is not trivial to do the coupling in the cpp-standalone mode because many Python features are not supported. I am not an expert in cpp, I do not have enough time and ability to fix the potential bugs. So I prefer a time-consuming but lazy way, that is cython.
Also, note that the most time-consuming part is the repeated restart of computation due to coupling, instead of choice of cython/cpp.