They don't pay that well relative to the local market, insist you be in SF at the same time and only want full time. They can solve the problem by allowing remotes, having a few part timers or paying the SF standard salary. If you try to do a simple github pull request on the repo, it takes a very long time to do a code & design review so it makes possible maintainers discouraged from contributing.
It's the old choice triangle: fast, cheap, good: pick two.
Here is the tracking issue for data usage.
https://github.com/WhisperSystems/Signal-iOS/issues/866