It's simple, Earth was a discovered planet that had all the rights components to support life. But there was one problem, life on Earth was threatening. Dinosaurs were rampant, atmosphere was too dense and it would be impossible for intelligent beings to live on the surface of the planet.
Because destroying all life on a planet was prohibited, they still decided to land and try to live below the surface. Many died of starvation, low oxygen, disease, eaten alive by dinosaurs so they went back in space and came up with a plan. Take the DNA of many of the animals of their birth planet and hide it a huge meteorite. Then have that huge ass meteorite crash on Earth. This would wipe all life on the planet in an instant. Because a huge cloud would prevent the sun from bringing back life on the planet for some time, the DNA and seeds from the meteorite would flourish and bring new species on the planet. This would also prevent any native life from Earth to ever live again and make sure the new kings, the Mammals reign supreme on Earth.
They were found guilty though and were prevented from ever landing on the planet. Earth was banned from any touristic and business destination until it regains it's previous state. Things though didn't go as planned, humans appeared and they didn't look like them, but they are no threat, they think Earth is flat and don't go to space... oh wait!!!
