A dog and a pig attempt to mate. we are aware that these are two different animal species. but what is the best explanation for the barrier that makes it impossible for them to successfully mate?

Dog and pig belongs to different species. The male reproductive cell or male gamete or sperm cannot fertilize with the female reproductive cell or female gamete or ova due to prezygotic barrier. The barrier could prevent fertilization, so that a zygote cannot be formed to produce hybrid.