The iPhone has about a 56% market share in the US, compared to a 26% market share in Europe (with the UK being the outsider with 52% market share).

That makes a huge difference: in Europe it's very unlikely that everyone out of any group of friends has an iPhone, making Whatsapp, Telegram etc the goto solutions for group messages.

Apple is also huge in Japan, but I don't know what kind of chat apps are used there.