The tl;dr is we had absolutely no good business reason to choose Scala, and very few technical reasons. It is mostly a developer happiness gamble.

A secret ulterior motive of mine was that Scala has bad support for Spring. Every other Java service at the company uses Spring, and I abhor it, so if I can drive adoption of Scala on my team then we can avoid Spring and also provide pressure to make all our shared JVM libs Spring-agnostic (which I believe is a good move in general).