Privacy

Well, it's big pretty much everywhere except for the US and China. It was the right app at the right time and due to its huge user base it's virtually impossible to get people to ditch it - even when it was bought by an evil tech giant.

It does have good and useful features though. SMS doesn't even allow group chats - that alone renders it utterly useless (I'm just ignoring it being clear text without any kind of security or privacy - those seem to be too abstract concepts for most people anyway).