Presumably, Tumblr also uses Scala for the services themselves... and by assuming Scala, they can make the API more idiomatic.

(For me, the question runs the other way -- I need a very good reason to write a codebase in Java vs. Scala these days, although I understand not everyone feels the same.)

A good enough occasion to learn the language?

Not sure if "learning opportunity" should necessarily be on the priority list for a framework that apparently is intended to serve as the backbone for a lot of service development within a company the size of Tumblr ;)