SMS isn't a great option if you're communicating with people in multiple countries, and often not if you're traveling internationally yourself. Signal might be, if only we could get everybody to use it.
Back when it made sense to send MMS messages (i.e. when everybody haven’t had something like WhatsApp) they were ridiculously expensive while sms was free on most plans. Nowadays it’s different but for example on my plan, sending MMS while on roaming is not plain data depending on the destination.
The criteria was that everyone needs to have access to the messaging platform. SMS checks that box. SMS does not do literally everything, but that's not the most important quality of a messaging platform.