Because the united states believed that it was its well... "destiny" to go out in the world or at least west, and take what ever land was there, as the united states was the great liberator for the people who wanted to be free- so it was its destiny to grow its borders, anyways, this made adapting new states into the union almost policy and the united states was eager to add more land and territory to its borders.
Maybe right depends on the subject but a major study shows that poor countries have higher birthrates but in first world countries there is less thats is why there is less babies being born in the 1st world countries because of the better health care and access to cleaner water and more food and economic growth and lesser corruption so the awnser would be economic growth or you can rephrase the words i used to make an awser that fits hopefully i helped
