The most likely reason is usually because the lands and resources are available
In order to survive, human have to fulfill a certain needs such as foods, cloths, and shelter and if a certain area couldn't provide that ( let's put Sahara Dessert for example) , the only logical thing to do in order to survive is to move to the Area that can fulfill it
1 ly is a distance, defined as the distance travelled by light in a year. hence 2.5 mn lys away means it takes 2.5mn years for light to reach earth b/c light travels a distance of 1 light year in a year
