There is no logical way to verify that all activists (or even a majority of them) use Signal over WhatsApp. The perception that activists use Signal may have been enough to block them, but having a huge backdoor in WhatsApp is reason enough to not take action.
That's assuming it was a macro decision, and not a micro decision. The govt could have had specific intel on a particular activist, or cell that they knew were using Signal, and shut it down to deal with that situation at that time.
Signal actively promotes as activist messenger by using names of revolutionaries and anarchists (Makhno, Proudhon, Masha Kolenkina) all over their website. Just for example: https://whispersystems.org/blog/images/signal-faces.png
