Why can't different species of animals breed together?

Animals that could breed but don't are often defined as different species. For example Darwin's finches are often divided into 13 species. Yet hybridization is observed where the large beck ground finch interbreeds with the medium ground finch. Also Lions and Tigers which are defined as different species also interbreed, but not in nature as their habitats do not intersect.

Horses and Donkey's do not produce fertile offspring because of differences in the DNA. The greater the differences in the DNA the greater the level of speciation.

The great leap of faith of Darwin evolution is that small changes in species can result in the manufacture of new species that are of an entirely different kind of organism.

Different species of animals can't breed together because they have differences in their genetic makeup, reproductive systems, and behaviors that prevent successful mating and production of viable offspring. These differences can include incompatible reproductive organs, differing numbers of chromosomes, and genetic barriers that prevent hybridization.