It was consensus between colleagues/friends. And because you still need to trust open source software as well. I certainly don't have the time to review the code, and the "somebody else will do a thorough code review" fallacy should be well known by know. As said, Threema at least had a name attached to it that I somewhat trust.

Telegram for example doesn't exactly tick the "not as questionable" box for me, not least because it isn't entirely open source either, but also because I always find it weird if someone is pouring money into software that is available for free, with no obvious benefit for them in return.