It could be, but much more people use WhatsApp in Russia, people like my parents who haven't noticed any laws against the free internet before.
They don't touch Signal or anything else as well.
WhatsApp has lots of clients for the shitty phones poor people use and a ton of Russians use WhatsApp. It's also possible that some implementations of these WhatsApp apps or the phones that host them are hacked, while others are not so it could be less of a binary distinction than one might assume.