FreeCol is a relatively old project (I think I recall giving it a spin way back in 2006 or 2007), dating back to when SourceForge was not-evil and that's where everybody was.
At the time, I'm not sure there really were better options than Java for a cross-platform open-source game like this. Maybe Python with PyGame.
Qt became available under GPL in 2000, so that would have been a potential match. Anyway, technology choices are of course very dependent on the project founders' skills and preferences.
At the time, I'm not sure there really were better options than Java for a cross-platform open-source game like this. Maybe Python with PyGame.