Well, the main reason is that, traditionally, good is pictured as a small force of elites whereas evil is seen as countless hordes of weaker creatures. The quality vs quantity so to speak.
Paladins often appear near-perfect (in several ways such as fighting, ethics and hygene) and are one of the better examples of the quality. Get the forces of evil paladins, and you'll upset the balance.
At least, that's how I heard it be explained
