- It's the least common dominator to chat with anyone with a Phone (if you have a smartphone, you can go MMS group chat).

- I am not giving another company personal data (Facebook, Google, etc.)

Ideally, I would prefer to run my own server for a chat program (i.e. Matrix, Nextcloud Chat), then I know where the traffic is coming to and from, and I have a lot of confidence that personal data isn't being leaked.

My second option would probably be Signal, as they seem to do to great lengths to avoid collecting data on you. However, trying to convince family/friends to use either is an uphill battle (a great deal of my family goes "why don't you just get an iPhone?"). So the pragmatist in me just uses MMS for group chat.