The iPhone is the most popular phone in the USA and iMessage integrates perfectly with iOS so when everyone you know uses iMessage and it's so easy to use then where is the incentive to use something else? It's the same reason why WhatsApp is so popular outside the US, when everyone uses WhatsApp then you end up using WhatsApp but in the case of the USA it's iMessage; providing you have an iPhone and if you don't then there is a good chance you'll be excluded from group chats (especially if your a teenager) and that's the whole reason why Google is pushing so hard for RCS.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 not a matter of superiority, it's just a matter of what everyone in your group be it family or friends use and in the USA for a majority of people that's iMessage.