Welcome to rootschat Darsy.
I can think of a couple of reasons, but there will be others. I'm sure someone will be able to provide you with some reading matter.
Adventure, promise of more opportunities and a new life , employment in the new lands, lack of employment at home .... Just the usual things.
Unsure if it was the case this early but I know that later some of the colonies were in need of people to populate and work on specific projects - schemes such as free passage, accommodation and employment were offered to entice (often) good protestant stock. Often these were advertised in the newspapers. I think the fact that you mention that they did well and became land owners may be the clue with your family, as land may have been promised to them. It must have been an offer too good to refuse for some (possibly) poor tenant farmers. There may have been a 'deal' whereby they were given land on the condition that it be cleared and farmed in order to feed the growing population. Pure speculation on my part of course though I know that this kind of thing did happen in some colonies at certain times.
PS. Often family members went together or sometimes one went ahead and called for the others once settled, so immigrants were not always alone without family.