Answer:
Because they are sterile
Step-by-step explanation:
Mules are considered to be a weak hybrid and not a separate species because they are sterile and cannot produce progeny of their own.
Mules are hybrid products from two separate species - a donkey and a horse.
A species is generally defined as a group of organisms that are capable of breeding to produce fertile offspring. Hence, in order for them to have the potential of being categorized as a separate species, they must be able to produce their own progeny but this is not the case.