It's not, it's a superficial opinion on color and functionality that apple and Google want you to think is a big deal but in reality it's two companies using two different protocols and they want you to pick up their product over the over. Even Americans don't know why it's a big deal, they see green in iMessage and freak out.
Sounds like you're an iPhone user, or at least someone who doesn't have kids with Android phones and doesn't participate in group activities where the people primarily use iPhones.
I won't bother explaining it all, but the real issues are group chats that contain both iPhones and Androids, and image/video quality, reactions, etc. The issues are bad enough that kids especially get left out of group chats over it, and bullying incidents are widely reported (and I've seen it first hand)
As an android user with a wife and kids that have iPhone, it's obnoxious that any video my wife and kids attach to a group message gets auto-reduced to sub-potato quality. We send them with Facebook messenger now but I can see where other groups might simply exclude the people with Androids to avoid the need to use alternate apps. Apple k ows what it's doing.
Mostly because the iPhone's dominance has led to iMessage being a very popular service over here. But Android users on the receiving end of messages from that service get images and videos in much lower quality, among many other quality losses.
I mean it's kinda a chicken and egg problem but I'd argue that you've got it the wrong way around. I think the iPhone is so dominant only because of iMessage. It's a status symbol to have blue bubbles, and there are people seriously not dating Android users for example.
The iPhone got popular for different reasons but the only reason why it's so dominant in the states is that people started viewing blue bubbles as something desirable. And that is by design! Apple has deliberately made texting between iPhones and Android phones shit by not updating to newer official texting standards. They stuck with an SMS/MMS protocol from the 90s and early 00s, just so that people keep buying iPhones.
There are lots of factors going into the dominance of the iPhone in the US. Platform lock-in, the ecosystem, the lack of an alternative with similar levels of polish for several of the early years, the mystique, the design, the build quality, Android's formerly fractured lineup, the rise of the iPad, app development trends that for a while preferred iPhone first; and, yes, iMessage.
All of those have their own complicated and oroboros-like causes and effects, with some being force multipliers, meaning that pointing at any one thing as the cause for any other is a little bit of a fool's errand, so I'll just say they're definitely related.