I believe its because they had trouble finding a way to make the money in their new settlements. It was different for settlements in different areas. Some lived in areas were the soil wasn't rich enough or too rocky while others lived in swampy areas and didn't know how to farm in the new soil from the Americas (goes for all settlements) An example is James town in which they weren't prepared and their population dropped greatly after the winter. it took them years to finally find a good cash crop (tobacco) which they used to repay their founder.
Of course, the answers vary according to what they taught you in class.This is what we learned a week or 2 ago
-Calypso, 8th, Advanced US History