Question

Why are common names not a good reference to a species

Answer 1

Scientists stay away from names common names because they are often not speficic  to a particular such as one species. Biologist uses these specific names to identify the different species.

Answer 2

Because different places have different names, even if they all speak English. In the U.S. we say "ladybug" while in the U.K. they say "ladybird" Scientific names are the same everywhere.