Probably because Common Lisp has a bad rap at NASA.

http://www.flownet.com/ron/jpl-lisp.html

Not that it answers your question, but JS dialects can be found in other strange embedded places such as Cognex's DataMan industrial barcode readers.

probably so astronomers can easily focus on their work - pointing the 'scope at stuff - and not worry about writing elegant/concise/reusable/succinct/$(feelgood adjective) code. I could understand it being used that way if it were for a limited scope (no pun intended) of operations, such as direction, focus, camera/sensor settings, etc. I'm sure they're not using JS to control thrusters or calculate trajectories.

So the Russians can hack it to self-destruct, obviously.