How come all of the animals in Jurassic Park can't breed?

Because they’re all female ... but later in the movie we see that they do end up breeding because the scientists filled in the dna gaps with frog dna; Supposedly, African Frogs that live in the same sex environment seem to change sex which allows them to breed. It’s what’s said in the movie ... hope this answers your question.

It is most likely because they were crafted using the DNA found in the bones of dinosaurs. So, in other words, scientists created them, and most likely the DNA did not give enough information for the dinosaurs to be able to reproduce.