Signal (Open Whispers) is likely CIA funded. It's like TOR (made by the Navy/DARPA): make something that works so your own operatives can hide behind it too
Because then you get a fight. Signal moves to F-Droid and F-Droid gets a huge mindshare increase and much harder to kill.
Signal is today's thing the security state in Europe & the US clearly hate and want to backdoor and destroy. So let's speculate they'd rather be able to make sure that no app, for any purpose that they don't control can survive or succeed?
1) Put google in control of what you can install.
2) Get google to block it.
Noting that making it harder to install does most of the job as you need you contacts to use signal before you can.