Let's see. In many countries Signal is now the primary communication tool for politicians. They love to communicate securely, without the risk of getting caught by law enforcement ;)
Signal is there. And you can also install it on private devices, that's what politicians like even more. That even their own intelligence agency can't read the messages.
A lot of politicians are corrupt, and they don't like laws that make it harder for them.
citation please
This was the beginning. I know from corruption cases in Austria, that a lot of politicians were using Signal. You can find it in the documents of the prosecution.