Because it was created to be a secure replacement for text messaging. I gather that in some parts of the world people don't use text messages so much anymore, s

Because it was created to be a secure replacement for text messaging. I gather that in some parts of the world people don't use text messages so much anymore, so this may not seem obvious, but in the US, Signal was primarily adopted as a secure alternative to SMS. People who routinely texted each other's phone numbers could simply install Signal and continue doing exactly that, with the added benefit of encryption.