Antipater was a business man, born in Macedonian that had fought for Alexander´s Father, the king Phillip II of MAcedon, and for Alexander as well, when Alexander wanted to face east and conquer Persia, Antipater was left as Greece and Macedonia´s regent, which basically gave him alexander´s powers to rule in his absence, when Alexander died, he kept those powers as regent and ruler.
There are more than three reasons why this happened of course but the best three are there they needed more land, resources, and some way to escape the cold.
