Threema is a pay-to-play client on the App stores ($3 USD on Google Play, €4 on the Apple store), inhibiting it's adoption when the playing field is full of free clients. The inability to try-before-buy (regardless of technical merit) turns away casual users who might give it a shot "just because", myself included. (Theory: the world has been burnt a thousand times over by shitty ad-infested mobile apps and no longer trusts buying before trying. I sure don't.)