Answer:
Because the parents (male horse and female donkey) are different species.
Step-by-step explanation:
A mule is an organism produced as a result of the fertilization of a donkey's egg by a horse's sperm. The parents of this hybrid animal (horse and a donkey) are two different species of animals with different number of chromosomes in their cells. The horse and donkey contains 64 and 62 chromosomes in their cells respectively.
These chromosomes numbers are reduced to 32 and 31 in the gametes of the horse and donkey respectively. However, when these gametes come together, they form a mule with 63 number of chromosomes (32+31). The two chromosomes of the different animal species form an hybrid that is different from both of the parents. Hence, the mule doesn't resemble the horse or donkey because the chromosomal (genetic) combination is different.