The decision behind choosing a server side technology is never an easy one. Every single language has it's pro's and con's around each.
For me, this decision came down to a couple simple points: 1. Node is a web tech first class citizen, designed around handling web events, in a web technology world 2. Asynchronous
The thing about Python and Java is that they TOO can handle these, and handle these very well. Java for instance powers most of Twitter and Netflix's architecture. Where Python is what's behind giants like Instagram and Patreon. Certainly, you can't go wrong. Heck, Ruby powered GitHub and GitLab, and those things see HUGE traffic.
But this project is a web technology first. And node feels right at home as it itself is a web technology. This decision was more about homogeneous synergy than most anything else. I need it to be screaming fast, asynchronous, and play extremely well with web standards.
Node fits the bill on every front.