Why do newborns get constipated ?

Many reasons: iron containing formula is a common reason. Breast feeding babies mostly do much better

Anatomic: Infants are prone to rectal sphincter incoordination (haven't developed the tendency to relax during stooling) which results in straining and grunting, often misinterpreted as constipation. Infrequent stools are common in breast fed infants (stools are still soft). True constipation (hard, infrequent bowel movements) is uncommon in infants and usually due to soy formulas or inappropriate mixing

Several Reasons: By the time the baby goes home from the hospital we like to see several meconium stools daily. In the first week of life constipation can be related to relative dehydration in anticipation of mom's milk coming in. There are other more rare causes that are associated with abdominal distension, so that is a symptom to watch for.