I'm pretty sure it was the horse bc native americans no longer traveled on foot for long distances. instead, they had the ability to travel using horses which was faster and more efficient
They searched for El Dorado one of the infamous cities in the andes.
Economic sanctions and military force
The west offered new settelers free land
(Not related just helpful. can't not is a double negative. "I can't not leave'' means I can leave)
The biggest reason is probably we can even travel to Mars. Mars is only one planet away from us. the distance is to far for fuel and time. we would need to go much faster and a lot less fuel just to get to Mars.