I was not a first party to the discussion, but what I heard is that Alex Roetter had asked the most senior engineers to consider strongly whether or not it would be better to have one JVM language. Twitter pays a polyglot tax and Scala's tool chain is clearly not as mature as Java's. In the resulting discussion, though, even engineers that primarily wrote systems in Java wanted to keep Scala as an option.

I had one conversation 5 years ago with Raffi about which language something should be built in. He advocated strongly for me doing it in whatever I felt made the most sense. I think this is typical of his management style -- which is to establish goals and let the people responsible for achieving those goals work out how to do it. Raffi had left the company by the time the JVM standardization discussion happened.