I’m in my forties so I was around when cell phones first became a thing and you had to T9 type your messages and was in my late twenties when smartphones became a things. The cost is the right answer. It was much cheaper in the states to txt earlier than other places. So the US stuck with SMS longer as that’s what people were used to and it eventually became free while in other parts of the world it did not but data and WiFi became more affordable, so people jumped to IM.