Land and opportunity. In most cases a poor person with no prospects could go to a colony and just take any "unclaimed" land or resources for their own.

Land, because their former countries were already very cramped and it gets harder and harder for people to make a living like most people did back then.

They were also penal colonies, often times criminals would be forced to go, and in the case of Jamestown colony, it was often the case that young men on the street were often taken against their will and just put on a boat where they were forced to colonize (one reason Jamestown had such a hard start). And some people just did it for the sense of exploration, to escape from their pasts, or even escape from the law.

(Source - American Colonies - Alan Taylor_