Well, I am likely misguided since I never wrote a compiler in my life -- but to me it feels the "purer" a language is, the easier it is to make automated analysis on how to parallelize its code fragments? Hence FP languages are better candidates, I'd think?
It can be a helpful feature together with other language design features supporting parallelism but it's not a silver bullet. Existing old sequential world PLs have proven hard to auto parallelize, FP or not.
