My 2c is that R has a wealth of libraries and utilities (for statistics, machine learning, finance, etc.) that are not as easily available for other languages. Things like scikit-learn for Python close some of the gap, but looking at packages on http://cran.r-project.org/, it seems that Python might have some catching up to do.

I make no representation as to the quality of the libs available for R, as I don't have a whole lot of experience with it yet.