Why does the spinal cord enlarge at cervical and lumbar region

Because those are the levels of the spinal cord that deal with the innervation for the arms (cervical) and legs (lumbar). There is more mass of nerve cells, bodies, and tracts in order to accommodate the innervation needs of the limbs.
The normal cervical enlargement extends from about the C3 to D2 vertebra with a maximum circumference of about 38 mm. An analogous region in the lower limbs occurs at the lumbar enlargement termed as conus.