TL;DR: Apple dominates the US smartphone market, but EU regulations may offer Android a chance for resurgence by enforcing messaging interoperability and standardizing hardware features.
The reason is that messaging services like WhatsApp became popular in Europe because carriers charged exorbitant fees for SMS messaging at a time when no single phone manufacturer absolutely dominated the market. Apps like WhatsApp made it possible to communicate with people, no matter which specific phone or brand or platform they were using.
If the iPhone (with iMessage pre-installed) had been the dominant smartphone and ecosystem at the time, chances are that what's happening in the US would have happened in Europe in exactly the same way.
It's exactly the same argument as with Windows and Internet Explorer: if Windows had been one podunk operating system out of many, nobody would have cared. The whole issue was that Microsoft used the market dominance of Windows to quasi-lock users into Internet Explorer.