I am unfamiliar with scala, which of it's traits makes it deserving of higher pay?

I guess the fact that it's rarer than other languages, but I have no idea.

Just a guess: perhaps that it’s more applicable to large use cases and at clients that are much more likely to be profitable and therefore able to pay?

Scala is big in the financial sector, which means average salaries for Scala developers are going to trend higher.

Functional languages seem to have a smaller talent pool and higher salaries.

You can write procedural Scala, but if that’s the case, you might as well use modern Java.

It's popular in finance, which pulls up the average pay.

Also in all sorts of data engineering, productionized data science, etc.