It's easy. Even if most of your friends use iOS, that means some don't. And some may switch to Android. Nobody like having to use multiple messenger apps. People want everything in one place. That makes iMessages really unattractive since Apple doesn't offer an Android app.
In Europe, it's 30% iOS, 69% Android, 1% other. iMessages doesn't even reach a third of all users.
In the US, it's 42% iOS, 58% Android. Not surprising iMessages is more attractive here.
People in Europe mainly use WhatsApp basically because they were first to the party and people don't care enough about privacy. There are better apps now (Signal, Telegram) and some people and their friends groups are slowly switching over but WhatsApp is still by far the biggest one and it will probably stay that way in the forseeable future.