iMessage came after WhatsApp (WA itself isn't the first messaging app...), I wonder why WA didn't get big in the US, maybe it's because virtually all mobile contracts in the US include a texting package. As for WA, one key to its success was probably the automatic upload of your address book to see who of your contacts is already in the system (and the privacy side in the battle of privacy vs convenience loses another battle), on other IM clients you had to add contacts manually.
There are texting packages in European contracts as well, particularly in the more competitive markets in North Europe, but one reason to prefer WhatsApp is roaming: in the US, you can use your texting package all over the continent, but in Europe, you couldn't - whenever in a neighbour country, sending an SMS or an MMS could be very expensive, and what's worse, the price is unpredictable or even scammy.
So it was better to use wifi and WhatsApp to communicate. Once you'd fixed your net access, you could be sure of cheap easy text communications.
The EU roaming pricing law has now fixed the original cause, but it is too late to change people's behaviour.
IIRC WhatsApp also cared a lot about making the app available on all kinds of phones. Not just iOS/Android. So all those people in the rest of the world could start using it and bring their families on board.
WA is also practically non-existent in Denmark. If someone is one WA they are either an ex-pat or have lived extensive time abroad and have friends / family they keep in touch with.
Here we also have unlimited texting in all but the most limited pay-as-you-go offerings. The telco market is extremely competitive.
