I use SMS specifically because of the network effect. Almost everyone I want to communicate with has a cellphone, so all of them are capable of sending and receiving text messages. There is no need to list my installed messaging apps and compare it to theirs, we just need to exchange phone numbers.

My longest and most active conversations (chatting with my wife or my closest friends) are going to be over a more feature rich app, but if I need to set up a quick chat with 3 or 4 people, I am not going to think what messaging app we all have in common, I will just start an SMS group chat.