Their competitors are free. Charging $2 would make Signal a non-alternative for many of their users, and due to how the network effect works, it would greatly reduce the utility for everybody willing to pay as well.
And that's all without even considering the significant overhead of collecting low-value payments internationally.
If you have a sustainable business model, you don't need the network effects. Threema is fine with a smaller userbase because they have a business model that works.
And that's all without even considering the significant overhead of collecting low-value payments internationally.