It's not a question of "superiority" - it's simply availability. iMessage/SMS works well enough - what advantage do "other messaging platforms" really provide over this?Most users in Europe have long moved on to other messaging platforms. It boggles the mind people in the US still use SMS or that they think iMessage is in any way superior to other messaging platforms.
It's the same reason I use Safari and don't install Chrome. They are both web browsers and Safari comes with the OS.