As @scooke and @mehdi already mentioned, the sole reason to support only one distribution is the reduced complexity by having to deal with only a single well known system. Even different ubuntu versions require quite a lot of testing and different code. Sometimes package names change for example. We essentially just settled on Ubuntu 16.04 initially since that was available on basically every VPS provider. Really that is the main concern for us. Naturally for security updates we then progressed to 18.04 and will soon hopefully support 20.04. Other distros with even other package managers are simply just overhead and cause trouble to users. In the end from what Cloudron requires, there will hardly be any noticeable difference in speed or resources use between any of the distros, so we lean towards stability here.