The reasons why dogs have only one testicle may be:

Genetic factors. Some dogs are born with only one testicle, possibly due to genetic factors.

Castration surgery. The dog underwent castration, a common dog neutering procedure in which one of the testicles is removed for contraceptive purposes.

Unilateral testicular hypoplasia. A dog's testicles are located in the abdominal cavity at birth. As the dog grows, the testicles will gradually descend toward the scrotum. Most dogs have only one testicle that descends into the scrotum. This condition is called unilateral testicular agenesis. , is one of the common reproductive problems in dogs.