People use Python for finance and time series because of how easy it to read in the data. There is a library called pandas which allows you to read in a txt, csv, or excel file easily and do any type of array manipulation you want. The pandas library is written in C++, so it's basically a python wrapper.

Conversely I don't think C++ has this same type of library for reading in data in a single line.

While C++ is faster than Python overall, for most financial simulations Python is definitely fast enough.