The server was not always opensource and until the phone part moved from redphone's implementation to signal protocol, that was also proprietary. Additionally,

The server was not always opensource and until the phone part moved from redphone's implementation to signal protocol, that was also proprietary. Additionally, they have been hostile to federating, 3rd party appstores (fdroid), and until the past year or two it was not possible to run the android client without Google play services and analytics.