Scipy and Numpy blow anything Ruby has completely out of the water.

Python is significantly faster than Ruby, and the gap grows when you consider pypy. Also, simple historical reasons -- Python was first to the scene (if you exclude Perl), so it has more scientific libraries. Ruby seems to be preferred in domains like sysadmin and web apps where there is little emphasis on performance, so performant libraries don't (generally) get developed in Ruby.

It's been around a while, has an easy to follow syntax and has excellent libraries for science (SciPy, NumPy, matplotlib, etc).