So basically Valence is a developers' tool suite? Why they choose Javascript when are there already platforms like Lisk that offer a Javascript SDK?.
I didn't read the whitepaper yet btw
I guess because Java is the most used programming language.
I highly recommend to read the white paper! It should enlighten you! Otherwise feel free to ask.

They want to support every programming language, but imho it's very smart to release Javascript as first SDK for Valence!
Cheers