The only reason we switched to Python and Twisted for the first gen Matrix server (synapse) was for rapid prototyping using a platform that we reasoned the open source and selfhosting community would already have installed and be comfortable with. Java felt way too enterprisey and non-open-source-friendly, making quick tweaks to the codebase a huge pain, not to mention the verbosity of the language. The team agreed that expecting casual folks to install and use a JVM just for a chat server would be a major turn-off, and we continue to feel that was the right call.
It's also interesting to hear that a Java deployment was thought to be more difficult than a Python deployment. My baised opinion is the exact opposite. Java services are generally incredibly simple to run, especially if you make a fat jar executable.
A few circumstantial examples. If you look at the Matrix SDKs and implementations (https://matrix.org/docs/projects/try-matrix-now/), Java's presence is sorely lacking. The initial server implementation was written in Python of all things and the rewrite in Go. This is baffling as a Java implementation seems like a no-brainer to me.
In addition to working for a variety of corporations, I have also done some work for academic institutions, specifically in the library space. They have a lot of old tools from the 2000-2010ish range that are predominantly in Java. However, now, these institutions are primarily using Python and Javascript, and are even struggling to find Java developers to maintain their old infrastructure.