Why does cross breeding work between certain species but not all?

Your genetic code is like a recipe, a series of biological steps that describe how to make you you. Your unique recipe is created by going through your parents' recipes, and at each step choosing at random from your father or your mother. If both of your parents are German chocolate cakes, their recipes are going to be very similar and most the steps are are going to be the same. Maybe one uses a darker chocolate or has more pecans, but isn't going to matter too much which parent you get which step from. If one of your parents was a red velvet cake instead, the recipes are different, but still pretty close. There is pretty good chance you'll still get some sort of cake out of the combination, but it is going to be a pretty unusual one no one has ever seen before. But if one of your parents was a banana cream pie, those recipes are too different. When you try to combine them, you are probably just going to make a mess, and not get any sort of edible dessert out of your efforts.