The simplest answer to this question is because your child has brain damage. There are many possible answers to this second question, because there are many reasons children can sustain brain damage. Your doctor must carefully review your child's health history and conduct a variety of medical and neurological tests to help determine the cause.
Cerebral palsy is caused by an injury to the brain before, during, or shortly after birth.
In many cases, no one knows for sure what caused the brain injury or what may have been done to prevent the injury. A large number of factors which can injure the developing brain may produce cerebral palsy. A fetus's brain may fail to develop the usual number of brain cells, communication between brain cells may be impaired, or brain cells may not migrate to the areas they are supposed to. Causes of these malformations are frequently unknown, but can include genetic disorders, chromosome abnormalities with either too much or too little genetic material, or faulty blood supply to the brain. Developmental brain malformations in the areas of the brain which control voluntary movement may cause cerebral palsy.