So. Why make this a separate app? Why isn't this in Hangouts?
Hangouts almost nailed it, to be honest. For the people that I know who have both Hangouts and Whatsapp, we prefer Hangouts consistently - even for just text. The problem is that Google realised the importance of IM far too late and Whatsapp is now entrenched. Google is throwing solutions at the wall to see which sticks. It honestly doesn't seem like a bad attempt. Totally would have preferred it in Hangouts, though,…
Speaking for myself, I've found Hangouts to be a noticeable drain on my battery (iOS), while WhatsApp seems negligible.
I'm also on a smaller screen iPhone, and with the icon bar and the timestamp placement in Hangouts, I can read far fewer messages on the screen at the same time than with WhatsApp.
Personal preference, but I also like that WhatsApp uses the iOS emoji across platforms, so I know what emoji my recipient is getting – when I use an emoji in Hangouts, they get replaced by a Google emoji, and I never know what it's going to look like until I've already sent the message.
Finally, Hangouts doesn't use end-to-end encryption, which personally makes me feel very uncomfortable (I also didn't like using WhatsApp for a long time for the same reason).
This is anecdotal, but to me it's more than WhatsApp just being entrenched – in many ways, they really do have the better product.