I am curious to know: “Oranges” is a general term for many round citric fruits. Each one of them has a different scientific name, even when they are quite similar to eachother. On the other hand, dogs come in all sizes and shapes. A Rottweiler is a Canis lupus familiaris. A poodle is also a Canis lupus familiaris. Why don’t we give differetn scientific names to dogs, even though one weights 1 kg and the other one weights 60?
Because the dogs are the same species and the oranges aren’t. Don’t mean it to be skarky. The dogs can interbreed and are the same species. Their difference is the same as the difference between Shaquille O’Neal and Herve Villechaize.(of course those two can’t interbreed) The oranges are more like the difference between tigers and mountain lions.
For one thing, the BSC (biological species concept) really can only be applied to wild species, since it defines different species as those populations which do not normally mate and produce offspring in the wild. Coyotes and wolves, for example, are perfectly capable of reproducing, but they don’t do so very often in the wild.
The general rule of thumb for domesticated species is to lump them in with their nearest, wild relative. Dogs and wolves are Canis lupus. Sometimes, dogs will be put in a subspecies: Canis lupus familiaris. But breeds are only meaningful in the sense that are deliberately not allowed to reproduce with other breeds. A purebred lab isn’t part of a naturally occurring population, and has to be artificially constrained so as not to interbreed with other types of dogs.
Actually, all citrus fruits are inter-fertile
As the Wiki article notes, the actual species status of the named varieties of Citrus is obscure and controversial. Some may have been distinct biological species originally (which means they did not hybridize in nature, even if they may do so now in cultivation). Others are merely cultivars of the original biological species, or represent hybrids between two species. In the case of cultivated plants, many forms may have been given Latin species names in the past that would not presently be recognized as species under the Biological Species concept.
Linnaeus actually recognized many different species of domestic dogs. However, this was long before the Biological Species Concept was formulated. Under present species concepts, all domesticated forms are included within the ancestral wild species. All breeds of domestic dogs can interbreed, and are interfertile with gray wolves (as well as other members of the genus).
The different cultivars of orange are given names because its BIG BUSINESS… its important to decide the best cultivar to avoid disease and boost production , and ensure taste and quality…
But they are cultivar names and that means they are not called different species.
In fact, orange is declared to be one species name, Citrus × sinensis , which you might recognise as Citrus “Chinese apple” … due to its origins in China.
Any other species different enough to fall outside citrus seninsis is NOT orange, and that is why it has a different NAME ! Its not that the “orange” has a different species name, but this other species example has a different species name to be clear that is the case, and it is called “orange” in loose terms.
The oranges comes from the cross between two or a few citrus species
( perhaps pomelo and mandarin.)… has anyone does extensive genetic testing to verify which cultivars are crossed from what, ? the orange family tree ?
A standard scientific name usually gives the genus and species, such as Canis lupus. If a subspecies is specified, it will be given as a third word, such as Canis lupus familiaris. In the event that a classification finer than subspecies is desired, that’s denoted by the abbreviation var. before the name of the varietal name, such as Canis lupus familiaris, var. poodle. In practice, though, subspecies are already an extremely squishy concept, and even species isn’t defined as rigorously as we’d really like to pretend that it is, so it’s very seldom scientifically relevant what varietal something is.
It may be worth noting, too, that animal and plant taxonomies are governed via different sets of rules. For animals, the International Code for Zoological Nomenclature (ICZN) sets the rules for naming taxa; plants fall under the International Code of Nomenclature for algae, fungi, and plants (ICN) (which recently replaced the former code, the International Code for Botanical Nomenclature (ICBN)).
Because there are different codes governing the assignment of taxonomic names and categories for animals and plants, it’s not surprising that there may be some differences with respect to how “varieties” are handled between the two groups.
Well, then they aren’t the same species, are they? Or are they? What am I not understanding here?
Boy parts + boy parts =/= able to reproduce naturally. 
I think this should be emphasized more – the short answer to the OP’s question is that biological taxonomy is an ancient art of mixed usefulness. The “scientific name” system was started in the 1700s, long before we had a modern understanding of evolution and genetics and interbreedability and all that. It’s just a huge mess and we’re still trying to refine the definition of “species”.
So you have an old system that tried to match species based on limited understandings of their (best-guess) interfertility and visual similarities mixed in with new species identified through genetic similarities… it’s just messy.