I think a big reason for this is that in Europe people frequently move between countries, eg for school or work or vacation. This often means changing SIM cards and local phone number.
So it makes sense to use whatsapp because it's data based and you can keep the same account across SIM card changes.
That's actually a bug-feature: whatsapp is supposed to be tied to a phonenumber.
For example, take this use case: you have whatsapp with a german number. You go to italy. You get an italian sim. You get a new phone. Result: you can not use your german whatsapp. There is no way you can activate your german whatsapp account without yout german sim.
That's why you keep your original sim around (receiving an sms from abroad isn't very expensive and that's enough for re activating your account). And you can transfer your account to a different number as well.
So it makes sense to use whatsapp because it's data based and you can keep the same account across SIM card changes.