That's probably because AFAIK Apple doesn't allow process forking, making any Tor-based messenger almost impossible to run as Tor would have to run as part of the main thread.
>iOS doesn’t allow apps to fork subprocesses. While on the desktop Tor is running as a separate process, on iOS Tor is hacked to run as a thread inside the app itself. Therefore, you can’t have a system-wide Tor process like desktop and Android. If Tor is running in one app, and you open a different one, it’s not automagically going to start using Tor.
