[I'm an American] I've heard that the reason WhatsApp became so popular in Europe is because European cellphone services used to (still do?) charge for texts per message whereas in the US it quickly became commonplace for all service providers to include "unlimited texting" plans. From my perspective, I never felt the need to download any additional app for messaging - the default messaging app was always the go-to way to text people before smart phones, and then when the iPhone came out that just continued with iMessage as the new default messaging app.