Skype is a Vo IP service, which uses the Internet to allow people to make and receive free voice and video calls online for free or for cheap.
Whats App went one step further than Skype in a certain direction, that of identifying users on the network. Whats App widened its user base to include Windows Phones, Nokia and other phones, web, and desktop, and Jio (in India).
Also, 2G and 3G data were getting more accessible and cheaper in many places. Whats App launched at a time when people needed what it had to offer.
Although Whats App is a free app, data rates apply in some instances. It went unchallenged for a couple of years before real competition came around.
Soon, Whats App added video calling and recorded voice messages to its offerings.
Whats App was made for mobile devices and not for computers.