I love SMS as an interface (when I'm not talking to "AI"), but it can be risky for companies because 1) SMS/MMS messages are not free, and 2) sending them to consumers is heavily regulated in some places.
Also, it's my understanding that a lot of countries are mostly using messaging services (like Whatsapp) instead of SMS to talk to each other. In those places, SMS support wouldn't be as easy to get people to use.