There is confusion about the Quarantine, as this was not a one-day imposition such that on Tuesday, all must vacate Earth. The giant hominoids on the 12th Planet in fact are unaware that a Quarantine was imposed! What they became aware of was increasing problems while living on Earth. They considered Earth to be a swamp, in any case, full of carnivores their world does not have, and disease. They lost Gold shipments in heavy storms at sea, lost slaves and soldiers to accidents, and became disenchanted with living on Earth. They were already on Mars, and this became increasingly attractive. After leaving, in the main, they found mankind increasing in numbers and tinkering with technology, tool, etc. However, there were some who liked living on Earth, enjoyed pushing their slaves about and raping captive women, and were loath to leave. Thus, tales like David and Goliath have their roots in truth.