Earth was falling into chaos. Years of horrible natural disasters had left millions homeless worldwide and millions of others dead. In light of this, the world's space programs began desperately searching for ways to find ways for human life to live off of Earth. On a mission to see if, perhaps below the thick atmosphere of Venus, Earth's twin, that there might be at least a semi-habitable zone, they discovered humans already living there, on the surface. These humans did not seem to have the same level of technological advancement as the humans of Earth as most of their structures and boats appeared wooden and they lacked motorized transportation but rather seemed to lead life's that largely resembled that of medieval Europeans.
Upon further study, it was discovered that the fairly primitive dwellings often had a reliable indoor water source, drainage system and most communities had a decently advanced sewage system, meaning indoor "toilets". They would also see that most communities had communal outdoor "washing machines" reminiscent of an old Romanian style and grain mills for various types of grain.
Pretty soon, a small group of Earthborns was sent there to see if they could acclimate to the lives of the native peoples and then after a year, return to Earth to pass along their insights and knowledge to others. Some of these people ultimately decided to return to Venus to live longer stretches.
After 20 years of this back and forth, the earliest "permanent" settlers were sent to Venus. Among these early people were those in need of protection, such as those in Witness Protection Programs as Project World 2 was able to thoroughly vet each and every person switching planets and left each group on different parts of Venus, for further protection.