Why is lockjaw common in babies?

Cultural practices: In the third world many are unimmunized for their lifetime and moms don't pass any transient resistance to tetanus germs on to their newborns. Unsanitary cutting tools & or the practice of putting cow dung on the cord stump to speed separation are practices that lead to neonatal tetanus.In industrialized societies, a mom that avoids the recomended tetanus booster may leave baby vulnerable.

Lock jaw: I don't think it is common. However, after trauma kids are more prone in to getting ankylosis of the joints.