I asked on twitter, and got a reasonable response. Investing in learning Scala is a much longer term thing than learning Go (which is, in their experience, a much quicker thing to do).
Because you always need to use all the features the language provides. Simpler -> limited.
C is much simpler than Scala, is it more limited?
In it's expressive ability - definitely. In terms of implementing stuff, everything that is Turing-complete could be used.
I'd far, far rather relearn Go than relearn Scala...
Given that Scala is far more complex, learning Go from scratch is probably a lesser task.
How did they come to choose Scalatra if nobody knows how to use Scala?