Ruby performance in general is getting much better and when C extensions are necessary (as they often are in any high level language) libraries like ruby-ffi have taken a lot of the pain out of that process.
If that's a challenge, good luck getting a lot of NotProgrammers to move to a completely different language.
Besides, if I may be blunt, who in Rubyland cares if these guys are on Python? By and large, these people are on an island. They are optimizing for things you pretty much don't care about, for programming styles you're not using, for libraries you've barely heard of. Trying to entice them off of Python onto Ruby is a waste of time for everybody. The Ruby community would at best get nothing, at worst net pain of supporting a lot of newbies, and the scientists would incur the horrifying overhead of a language shift for the closest thing to a sidegrade available in the set of the top 20-ish language. If they were going to do that they probably shouldn't be going for Ruby, they should probably be going for Haskell or OCaml, both languages for which I could outline actual advantages to the scientists for the pain of the switch (though probably not enough to justify the switch).
If you think the NumPy and SciPy stacks are pure python, you'll probably be surprised to find a lot of C and fortran sitting under the hood.
The fundamental issue I believe they all had is lack of community. There is a quite large community of folks using Python for scientific computing. Many are not professional developers but rather scientists / engineers who aren't as open to jumping around to different languages as a developer would be.
These issues combined with the fact that Ruby is mostly pigeon-holed as a web only language would make replicating the community that the NumPy/SciPy/matplotlib stack enjoys almost impossible.
It also doesn't help that such capability have to be implemented as C extensions due to performance requirements. This greatly limits the number of developers that would/could work on such a project.