Bad Moon is an excellent way of enhancing creatures, as is Unholy Strength. An aggressive deck tends to have less land than a slower deck, so it's harder to fit multiple colors into the mana supply. When playing a very active deck, I generally want to empty my hand in the first few turns-impossible if I'm missing a color. Also, playing only one color makes it easier to play Wasteland-one way to keep a slower deck from fighting back. Out of the blocks.
C onsider the very start of a game.
You might use Dark Ritual to play a Carnophage and a Dauthi Slayer, while your opponent's first spell is a Wall of Blossoms. Notice that this plan relies on getting a very consistent, early creature rush. Because every creature does significant damage, holding even one creature back in your hand allows you to put a threat out after the board is swept. Keep in mind that every creature in a speedy deck should be put in with the goal of keeping the pressure on your opponent. A Wall is an efficient blocker, but a Black Knight is usually good enough-and more valuable on offense.