1. Old history of using python for scripting linux, starting from mid-late 90ies, especially around clusters and HPC, i.e. where you find applied scientists in academia.

2. A full C API. Not especially good, but powerful enough. I don't think ruby is much different, but wrapping C with python, again for HPC (or GUI libs). Lua C API is definitely better.

3. Communities matter

4. Different people starting to talk to each other more and more, and right timings. First mid 90ies (numeric/numarray, use in astronomy, etc.), then early-mid 2000ies (NumPy/SciPy consolidation, matplotlib, ipython), then late 2000 with scikit learn, and pandas.

Also, while I know little about ruby, I fail to see how much better it is than python. Both are essentially the same w/ some minor syntactic differences, and different communities. I have never seen an example where ruby was much better than python or vice versa for non trivial code. I mean is RoR that much better than Django ? I doubt that stuff matters very much.