Because menstrual cycle is the removal of the inner lining of the uterus which is basically there to help prepare the body for the formation of foetus.
If the egg is fertilized then the foetus formation will take place and the inner lining won't shed off thus no menstrual cycle will take place.
  • Brainly User
The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. Menstruation only occurs if the released ovum is not fertilised. 
  Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. During pregnancy all events of the menstrual cycle stop and there is no menstruation.