A new report from The Information is now saying that Google was willing to beat the $19 billion offer from Facebook, even though Google apparently didn't really want to acquire WhatsApp. According to the report, Google was mostly trying to keep WhatsApp from being acquired by Facebook. Google CEO Larry Page also tried to convince WhatsApp to stay independent, in order to keep it away from Facebook.
In the end, Google apparently "did not seem excited" about purchasing WhatsApp. So, even though Google was willing to pay more than Facebook, maybe the lack of enthusiasm was what sold WhatsApp on choosing Facebook to make WhatsApp employees very rich.