Why is whatsapp not in the app store for iPhone?

I recently deleted whatsapp accidentally on my iPhone, and when I went to reinstall it, it wasn't available in the AppStore. I even went to the updates section and tried to locate it in purchased apps, and it wasn't there either ?! Not sure what to do. Please help. Also, when I search what's app in google and try to click then direct link that the company lists to download it in the app store, it states that it isn't available.