A while back I watched National Geographic's Science of Babies documentary and it was talking about mental abilities babies are born with. Such as being able to understand tones and fluctuations of any language. This ability disappears as it focuses on the language of it's parents.The brain builds up networks around the new information it's receiving leaving the other avenues undeveloped. I'm guessing it's the same with holding the breath. The brain is so focused on breathing, crying, laughing, talking... all involving breath, that it leaves the small branch of holding it's breath undeveloped and either forgotten or very hard to access.
If these sort of things are also interesting to you please watch the documentary it also claimed that babies understand simple mathematical concepts!