The Hearthstone meta consists of various types of decks keeping each other in check. Similar to the checks and balances of politics or the game rock-paper-scissors, Hearthstone decks have good and bad match-ups.
The overall easiest deck archetype is known as “aggro.” Aggro, short for aggressive, is a deck archetype that is centered around the idea of getting your opponent to zero health points as soon as possible. This is done through a typically low curve, meaning most or all of your cards cost very little mana. By flooding the board with cheap, aggressive minions, and constantly using those, your weapons and your spells to deal damage to your opponent, you hope to kill them before they can stabilize.
While the strategy of just going face and constantly hitting your opponent seems simple, playing against a deck that can clear your minions early and outlast you is the decks downfall. Aggro decks struggle against control decks because a control deck is able to clear aggressive minions and stabilize after the initial assault of an aggressive deck. With an advantageous initial hand, an aggressive deck can beat a control deck before their opponent can do anything about it, but on average, the control deck has the edge due to aggro simply running out of steam.
Although aggro decks struggle against control, they have the edge in the match-up with midrange decks. A midrange deck has some early and late-game cards, but focuses on mid-game minions in order to peak and win in the mid-game, rather than early or late. An aggressive deck can beat a midrange deck before they can stabilize and play their mid-sized minions, leading to an early victory.
Aggressive decks are efficient because win or lose, the game is over quickly. When trying to climb on ladder, even with a slightly lower win-rate than a control deck, because you will play more games, it is better to use an aggressive deck. This combined with aggressive decks typically being easy to play and inexpensive to craft are why aggro decks will always be relavent.
In the early days of Hearthstone, Face Hunter was the main culprit of aggressiveness. Over time it shifted to Shaman, Pirate Warrior and various other decks. Aggro decks used to try to win by turn eight or nine in order to not allow their opponents to even take a breath, but not anything after turn five is too slow. As time goes on, the meta will shift, but aggro will be here forever.