Didn't write the Standard but I assume in the case of Old English Games it's an extension of the history of the Pit Games they were created from. Modern Games however are a fanciers creation & were never used for fighting. In their case I assume it was an aesthetic decision. Undubbed Moderns lack the refined appearance of dubbed specimens.
You speak of "tiny little roos" so I assume you are referring to Bantam Games. Understand they were not fighting birds. They too were created for exhibition purposes.