The answer to this question has to do with the geography and climate of New England. These northern colonies were defined by two main elements that prevented farmers from growing and harvesting many crops. These were poor soil conditions and a short growing season.
The soil in New England is defined by being very rocky with only a small layer of fertile topsoil suitable for farming. This is the result of the last ice age, when the glaciers covered this land. When the glaciers retreated 10,000 to 15,000 years ago, they stripped much of the otherwise fertile soil from the land. They also deposited countless rocks and boulders known as glacial erratics all about. When the colonists arrived in the 1600s, they found it was grueling work to clear the fields of the stone debris. Even after doing so, they found that the soil was not fertile enough to grow much more than subsistence level amounts of food. (One notable exception to this is the Connecticut River Valley, which is characterized by fertile soil deposited by the river. In this area, cash crops such as tobacco and surplus crops were indeed grown.)
The other factor that limited agriculture is the climate of New England. Since the ground does not thaw until late March or April, farmers had to wait rather late into the season to plant their crops. These crops would then need to be harvested before the frosts came in November (or sometimes as early as October). This did not leave much time to grow their food. As a result, the New England growing season was not long enough to support large-scale farming.
Because large-scale farming could not take place in most parts of the New England colonies, farmers could usually only grow enough to support themselves with maybe just a little left over to sell or trade in their community. They had to rely on other enterprises to maintain their economy and as such relied on commerce, lumber, shipbuilding, and fishing.