That's not what's hard to understand. It's choosing to exclusively use iMessage knowing that not everyone can use it. And then shifting your social circle based on one company's decision to not support it on other platforms.
If you think that something extremely basic seems something that is hard to grasp for other people, it's usually because you're misunderstanding the situation, not because other people are failing to grasp the extremely basic thing.
Not sure why people having their own subjective views and preferences is such a hard concept to grasp.