This basically came down to two things: performance on compute-heavy tasks and a need for good tooling. We used to have a Meteor based Node.js application which worked great for RAD and getting a working prototype in a short time, but we felt pains trying to scale it, especially when doing anything involving crunching data, which Node sucks at. We also had bad experience with tooling support for doing large scale refactorings in Javascript compared to the best-in-class tools available for Java (IntelliJ). Given the heavy domain and very involved logic we wanted good tooling support to be able to do great refactorings that are just not possible in Javascript. Java is an old warhorse, but it performs fantastically and we have not regretted going down this route, avoiding "enterprise" smells and going as lightweight as we can, using Jdbi instead of Persistence API, a homegrown Actor Model library for massive concurrency, etc ...