Python is extremely popular in the scientific computing community because it seems to be really good for writing clear, easily maintainable code.  Also, most sysadmin tools in Linux appear to be built using Python these days.  It is a language that has acquired a large use base, and been tested a lot - also the amount of open-source libraries available is pretty impressive.  I cannot speak for the Armory devs, but Python seems like a natural choice (of course there are many others).