It’s the combination of features and the network effect iMessage has. That’s what makes it special to a lot of people. You get rich chat features (especially in group conversations) and you don’t have to negotiate what platform to use if everyone happens to have an iOS device.

Great for you if you live in an area where all of your friends have the same messaging app. They’re all a big step up from SMS, but there’s no messaging app I know that will reach everyone I want to talk to.

Other messaging apps have the same features as iMessage, but not the same user experience. For example, WhatsApp has a pretty massive spam and fake news problem, and Facebook messenger is significantly slower and applies a high degree of compression on media. iMessage offers the best UX, and that’s why, in my experience, iOS users will almost always opt to use iMessage over competitors when communicating with other iOS users.

No, because at least in the US, iMessage has one thing that no other messaging app has - install base.

End-to-end encryption by default and seamless multi-device support. It's a combination of features that's currently limited to iMessage.