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Explosion of interest in machine learning and data science and decent Python-based tools for those fields.

They followed up on that question with another blog post: https://stackoverflow.blog/2017/09/14/python-growing-quickly...

Python's huge popularity in scientific computing fields along with it being widely taught in schools. Notice also that R was on the list, which is obviously due to huge uptick in data science. R has been around for awhile as well.