I don't understand what Signal had to do SMS.

its mainly a Secure messaging app not an SMS client

And Telegram is the worst in terms of security and privacy.

It was analogous to the “Messages” app that iPhones have—a single, zero-decision place to go for texting, that opportunistically upgrades privacy without the user having to understand anything.

Casual users don't understand (and don't want to understand) the difference between SMS and other messaging services. And it makes sense - it's all a kind of chat, so why would they expect two different apps with the same core functionality?