Due to changing conditions on Europa, they could no longer survive there. The last survivors left to find a new home world in 1951. However, their spaceship crashed on Earth before they could leave the solar system. The humans who found them killed some and experimented on the rest out of fear. They were able to free themselves and control the humans, using them to help rebuild their ship.
Nearly thirty years after they crashed, they learned about the Doctor and tried to get him to help them leave Earth. In the original timeline, the Third Doctor aided them, but those events were reset when the Seventh Doctor was sent by unknown forces to replace him. Instead, their ship was destroyed and the surviving Europans killed by UNIT. (AUDIO: The Defectors)