User blog comment:Aandrew07/Discussion of the Week: Subspecies/@comment-1773866-20121023012615/@comment-4212769-20121024205427

Easily explained:

The time that an animal/monster needs to adapt to such a different environment would be over 100.000 years, at least. (Yeah, I'm bad at guessing so don't take the time I said too serious) In this time the Blangonga population that lived in the Snowy Mountains would have adapted even more to the Snowy Mountains and wouldn't be the Blangonga anymore that evolved to the Copper Blangonga.

And yeah, as Copper Blangonga isn't in any competition with the Common Blangonga it couldn't outcompete it due to the different environment or vice versa.