Most people in Latin America, and India have it. I also have plenty of African friends that use it. Not sure about Europe, China uses WeChat.
The reason is that cellphone carriers used to charge you by the message (I speak from experience in Mexico), this was expensive compared to the average wage. Cheaper to just connect to WiFi, and send as many messages as you want.
Also, WhatsApp was one of the first popular and reliable messaging systems for cellphones. If offline or with poor signal, your messages where enqueued, and sent when you had signal. Facebook nor Skype had this back a few years ago.
US is a rich country, SMS has been cheap since I've lived in the us (4 years). 3 hours of minimum wage, $30, for unlimited everything, data, SMS, calls. WhatsApp didn't really solve a problem.