Warning: The following contains SPOILERS for Legends of Tomorrow season 6, episode 14, "There Will Be Brood."
The penultimate episode of Legends of Tomorrow season 6 explained that no evil alien race in the Arrowverse had tried to conquer Earth until relatively recently because of the Fountain of Imperium. The mystic MacGuffin at the heart of the season 6 storyline was finally discovered in "There Will Be Brood," and revealed to be far more powerful and important than previously indicated.
Alien invasions are a frequent occurrence in the Arrowverse, with the first major crossover event (appropriately titled Invasion!) pitting the World's Finest DC superhero teams against the telepathic Dominators. Every season of Supergirl but one ended with some hostile species attempting to conquer the Earth or destroy it. More recently, the first season of Superman and Lois also centered around an effort to transform Earth into a new Krypton. This raises the question of why none of the many war-like alien races that inhabit the Arrowverse ever tried to conquer the Earth before the emergence of so many superheroes capable of protecting the planet.
The answer was revealed in the Legends of Tomorrow episode "There Will Be Brood," which found John Constantine still searching for a legendary alien artifact called the Fountain of Imperium, which was said to be a source of phenomenal power for any mage who could find it and prove worthy of its power. Betraying the Legends to join with the mad geneticist Bishop, Constantine discovered that the Fountain of Imperium was not a source of magical water like the Fountain of Youth, but the name of a colossal magic mushroom. Bishop further explained, as he recalled the stories he had learned about the Fountain's origins in his time, that it was a gift to the Earth from a race of benevolent aliens, which shrouded the Earth in a shield that made it invisible to invasive alien species.
The distinction of "invasive species" is an important one, given there is a long history of peaceable aliens with longer lifespans than humans living on Earth in the Arrowverse. To give one example, J'onn J'onzz, the Martian Manhunter, is nearly 700 years old and had lived on Earth for over 300 years before he adopted the identity of Hank Henshaw in 2005. Presumably, the Fountain of Imperium had no problem with allowing the Earth to be discovered by refugees who came searching for a new home.
It is unknown how long the Fountain of Imperium protected the Earth, but its days were numbered when John Constantine entered the picture. The blue-collar magus unwittingly sealed his doom and accidentally killed the Fountain in the final scene of "There Will Be Brood," leaving the Earth without mystic protection from aliens from 1925 onward. This paved the way for Bishop's sinister scheme heading into the season finale of Legends of Tomorrow season 6, as well as every other alien invasion of Earth in the history of the Arrowverse.