There is a variety of reasons cats lose their hair. The most common reason is allergies, particularly to fleas and over-grooming. When a cat gets itchy it will over-groom itself to the point that the cat will pull the hair out. Mange or lice also can make a cats skin itch. Also if the cat is in pain it will lick to help relieve the discomfort. An overactive thyroid can also cause hair loss.
Some cats have a hair loss on the outside of the ears called pinnae which is common in Siamese cats.
Some cats have alopecia which is a condition. Parasites that bring about mange, and fungal issues like ringworm, are also a common cause of alopecia.
Have your cat checked out by your vet.
Professional Pet Sitting Services offered in the Upper to Mid-Pinellas County Area