Because the EU specifically wrote a law requiring (among other people) Apple to support apps on their competitor's platform, as a response to a bunch of anti-competitive nonsense the tech industry does with messaging apps.
Apple might be correct in this regard, as iMessage is not very popular in EU specifically. For this to apply here, either Apple would have to be caught lying about their numbers (not impossible), the EU would have to lower the minimums, or the US would have to create their own counterpart as iMessage is much more popular in the US.
It's not even easy to know who's got an iPhone now: it's just a big phone with lots of cameras as any other one. However it's likely that two close friends, families, etc all using iPhones could decide to use iMessage for their own messages. Same as for... (had to google it) FaceTime.