One of the reasons is of course that Whatsapp is the most popular messaging system and people are always going through their Whatsapp chat boxes. So, there is a much more fair chance that they would read your messages, especially if the client has an interest in your company. Moreover, in Whatsapp, you can actually track if the reader has read your message or not (unless your client has restricted that option). In the case of ordinary messaging, you can only track the receipt of the message but not if the reader has read it or not. Moreover, often SMS messages are sent at a cost whereas WhatsApp messages are free.