The Guardians didn't know the Dark Master was a Purple Dragon

[CrystalBlazier]

So, in the cutscene after you defeat the Ice King in Dante's Freezer, Volteer says there were rumors of a purple dragons countless generations ago but were considered myths. That means Malefor's existence as a purple dragon wasn't written in public history and future generations had no idea that the Dark Master was one until DOTD.

Looks like Malefor's path to evil was so severe that the ancestors chose not to chronicle Malefor's existence in the history books.