Because that's what they're used to and that what works with all of their friends.

To be clear, I completely agree, and I wish they did use a cross-platform messenger. But the reality is that, in the US, the vast majority of iPhone users use iMessage primarily.

Because it is the default.

Yeah, but you’d think they could be bothered to switch so that all their friends can participate equally.

Especially considering the burdensome cost of absolutely free.

Half of the engineers I (want to) talk to refuse to switch from IRC, and the other half refuse to use it. In the end, I use both, and so do lots of other people.

Should we expect ordinary, non-technical users to behave substantially different? It's not clear to me that we should; most people find the status quo "good enough," and the activation energy for switching is higher than the minor inconvenience of some of your social circle having a slightly worse user experience.

Americans are lazy status-seeking morons is why.