Not everybody, and not infinitely far back. There was definitely a period where there were no free texts included (although I do think that by the iPhone introduction many did have it, but still not all!)

And a lot of people had to pay per SMS/minute! At least, for the plans that didn't require a contract or a huge load of money up front. Remember Net10, where the gimmick was prepaying for $.10/minute, and text messages around $.05 apiece?

And also because Whatsapp was available on every platform, from the dominant ones at the time (Nokia and Blackberry) to the newcomers (iOS, Android, even Windows Phone and more obscure ones like Samsung's whatever it was called).

wasn’t it the long distance thing? charged for sending SMS outside of your network or something. I recall whatsapp was the way to text your parents from one country in to another, even in the EU.

I know for a fact my fam adopted almost a decade ago, so we can text from USA to EU to SA to Canada. Family all over and it’s free.

because people were already uaing sms when it was paid beforehand before smartphones were a thing. people just used to whatever the default was, especially since its not like everyone switched to a smart phone immediately after the iphone 3gs' relase. sms was the default method to talk to people who were still using "dumbphones"

A part of it was that they put in the effort to support featurephones. People with smartphones always had other options.