I used Hangouts including the dogfood versions internally at Google. Problem was it was too complicated because it was designed by Googlers for Googlers. So it supported desktop and mobile, work email and personal email and phone numbers, text and video, and so on. In short, every single complexity conceivable was crammed into the app.
Whereas Whatsapp was simple - only phone numbers to sign up, only text and images, only mobile phones. That simplicity meant my parents could onboard smoothly and operate it without having to navigate a maze of UX. I literally saw Whatsapp winning in real time vs Hangouts and other alternatives.