Why can't an alligator and a crocodiles mate if a donkey and a horse can?

When we talk about animal ancestry, there is a tree of decent:

Kingdom: Superphylum: Phylum: Subphylum: Superclass: Class: Subclass: Superorder: Order: Suborder: Family: Subfamily: Tribe: Subtribe: Genus: Subgenus: Species: Subspecies

In order for two animals to produce offspring that are capable of reproducing, the animals must both be of the same species. Closly related species can mate and produce infertile offspring in some situations, but only if those species share the same genus.

When you think of interesting 'cross breeds' like the mule, liger, or wolf-dog, the animals are of difference species, but they share a common genus (i.e. horses and donkeys are both of the genus equus, lions and tigers are both of the genus panthera, wolves and domestic dogs are both of the genus canis, etc). Successful cross-genus mating does not occur in nature.

The reason crocodiles and alligators cannot mate is the same reason why humans and chimpanzees canont mate: we do not share the same genus. In fact, we're more closely related to chimpanzees than crocodiles are to alligators... We are of the same Subtribe as chimpanzees - crocodiles and alligators don't even share the same Family.

They cannot produce offspring together.