They've kept it simple, you don't need to fill up sign up forms and remember your user name. Your GSM SIM card is your identity.
Secondly, they tightly focused on SMS market. Didn't go for multi party video conferencing, screen sharing, and other seemingly cool stuff.

Majority of adoption was from oversees. Also it was used in a few large companies. The major reason was due to cross-platform solution on all mobile OSies. Applies text platform did not work well with Blackberry/Android/Windows

Built to be a “better SMS alternative,” according to the company’s website, WhatsApp allows for cross-platform messaging – across the globe – with users not having to pay for the texts they send and receive. Instead, WhatsApp “uses the same internet data plan that you use for email and browsing. Just like other products, apps earn money when they are purchased. For WhatsApp specifically, the app used to cost about $1 to download. Other users around the globe got to experience the app for free for year one, but then pay $1 for each year following. In 2016, WhatsApp scrapped the subscription fee altogether. Before 2013, those on the iPhone had to pay $1 as a one-time fee to download and use the app. It was then decided to put iPhone and Android users on the same level, making the app free for everyone, regardless of device, for the first year, and then $1 per year to follow.
Even “free apps” or those that are free to download can still make money, and lots of it. You’ve probably noticed advertisements on your favourite apps. Just like ads on social networks, in magazines, on TV, etc. companies pay for their products and services to be seen within mobile apps. While WhatsApp has not yet cashed in on this tactic, other SMS apps, like WeChat, which is well known in China, has both apps and online games. With only 440 million users, WeChat turned in $924 million in revenue in 2014's third quarter.
I mean, even if WhatsApp earned $1 per download on iOS and $1 per year on other platforms , and had a large user base, the company’s revenue might have only been hundreds of millions of dollars at the time of purchase. 450 million users at the time of purchase, growth opportunity, prospects, behavioural data, contact lists, keeping the app out of the hands of competitors. It is not about how the app makes money now.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath