SMS/MMS don't work or are unreliable for international communication. SMS/MMS, historically, have been unreliable cross-carrier domestically in the US. Many cheaper Android phones, especially, seemed to be particularly poor players with SMS/MMS group chats. SMS/MMS doesn't guarantee message delivery order (which can lead to some comical exchanges when the first split of the message shows up a day after the last split, but also obnoxious). MMS, depending on phone, isn't particularly useful for a lot of multimedia or mixed content messages (not just text or simple and small images).
WhatsApp is free on carriers in many countries (including large portions of Asia and South America), contrasting with other messaging platforms which use a standard data plan. It delivers messages reliably, group messages just work, and it works properly for mixed content messages. And data is (relatively) cheap in most countries where WhatsApp isn't free.