Their bot api is very easy to use. I have made some telegram bots because is so easy. Tried to do the same with WhatsApp and didn't even get permission and api access.
Average Joes use mainstream things, if you want to do something accessible, these bots are the way. Ideally, you want them to download or use an app that does some hand holding. I'm all up to the fediverse but let's be honest, it has some non intuitive things for today's user, most were trained to understand and like centralized services. They vaguely understand and use emails today, and mostly for account creation of other platforms.
Disclaimer: haven't tried to make signal bots, they are a thing?