It got acquired for the trust people put in the brand, not the technology or the employees. Facebook needed a backdoor into people's contact lists and WhatsApp had a huge user base of people who authorised it to access contacts.
More boring explanation: For most people Facebook is just a platform for connecting with other people. Any messaging platform with significant amount of users is a threat for them.
A platform like WhatsApp could evolve to cover the main things people are using Facebook for. History has shown that people are not too stuck on any platform.
