I think it has to do with the fact that the US (and Canada) very early on had unlimited texting plans, and iMessage would make use of that (iPhones just send texts for iMessages in absence of a data connection). WhatsApp can only use data, which is cheaper and used to be unlimited (that's gone) in the EU. In contrast, texting was and still is insanely expensive in the EU (per byte it is staggering even, compared to normal data). Since data coverage was poor in the US in the 2G days (is this true? In the EU 2G was GPRS, meaning available when normal voice was available... I think it differs for US networks?) and texting was often included in a subscription, iMessage made a lot if sense and was very reliable.
I remember early iPhone users complaining here in the EU that their iPhone unpredictably would sent iMessages either for free over wifi or for 50ct/message over sms/text (us EU citizens avoid sms/texting like the plague, allthough unlimited plans are now more commonplace, it's just too late). I recently set Signal to deal with my sms/text messages. Boy was that an expensive mistake when I accidentally texted my friend in Curacao instead of sending a normal Signal message.