In many ways a mule is a superior animal to the horse or donkey. they are often stronger and can jump higher than either of their parents. why then are they considered to be a "weak hybrid"?
they are sterile.
their offspring are progressively weaker.
they do not live as long.
they are more prone to disease.

They are sterile.

Explanation:

A mule is the offspring of a male donkey (jack) and a female horse (mare). Thus the number of chromosome of the donkey and the horse are quite different.  A combination of the horse chromosome which is (64) and a that of the  donkey chromosome which is (62) gives rise to 63 chromosomes in a Mule.

This major difference in number,forms and morphology typically prevents the chromosomes from pairing properly and producing successful embryos, as a result of this aftermath effect, it renders majority of the mules to be sterile.

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