People in scientific computing started using Python the mid-1990s. This included Numeric (ancestor to NumPy) which Jim Fulton, Jim Hugunin and others started in 1995. Ruby 0.95 wasn't released until the end of that year. Moreover, van Rossum tweaked Python so it would be a better fit for matrix computing, such as multi-dimensional slices.
Then there was PyFort by Paul Dubois at Lawrence Livermore National Lab (1999), and SWIG by Dave Beazley at Los Alamos National Lab, which had Python support by 1998 (see https://web.archive.org/web/19981212033200/http://www.swig.o... ). Those made it much easier to access existing scientific libraries through Python modules. (A phrase at the time was that Python would 'steer' the low-level high performance code.)
While at this time, Ruby was just becoming known in the English speaking world, and didn't really hit the mainstream until Ruby on Rails in 2005. This means Python had a 5-10 year head start, and Ruby hasn't caught up.
