They thought it was a learned response and mimicry. I remember my shock when I heard about it too. But they honestly thought that babies do so much developing between birth and two years of age, that their nerves couldn't possibly be the same as an adults.

Rather than perhaps thinking that children might in fact feel more because their bodies are at such a prime state of development and they're doing so much physical and mental growth.

They also thought that infants could not be traumatized by abuse since they're too young to recognize what was going on. That combined with believing that children didn't feel pain, is why child abuse for so long was about the ick factor and broken social contract involved for the adults instead of the pain and terror of the child.