I’m not sure that’s true. I’m fairly certain UK, France, AU, Canada WhatsApp is not vastly more popular than the blue bubble alternative. At least I believe this was the case a few years ago, based on data I’d seen.
Blue bubble isn't really a thing ever mentioned in France either, not enough iPhone market share.
Nobody uses iMessage. People with iPhone use WhatsApp too.
The user experience of iMessage used to be subpar and now everyone has WhatsApp installed anyway, the feature set is the same and it works on all phone brands so nobody feels like switching.