https://forum.syncthing.net/t/discontinuing-syncthing-android/23002

According to this post, it was partly that and lack of maintainers. Given there's maintainers for a fork, I'm curious why they didn't bring them into the main project.

Reason is a combination of Google making Play publishing something between hard and impossible and no active maintenance. The app saw no significant development for a long time and without Play releases I do no longer see enough benefit and/or have enough motivation to keep up the ongoing maintenance an app requires even without doing much, if any, changes.