The answer is likely politics. You can think of how irritated and angered colonists get at acts being passed that took a toll on their quality of life.
When the population is dissatisfied to the point of making a protest of great magnitude that overthrows the current system, a revolution occurs. Most revolutions occur for political dissatisfaction, sometimes for different reasons. For example, in the French Revolution there was dissatisfaction with the absolutist monarchy that sacrificed the population through abusive tax collection. In the Arab Spring of 2011, there was dissatisfaction with different dictatorial models of government that repressed individual freedoms.
Their climate helped them become the 'breadbasket' of British North America. Arable land was plentiful and the soil was fertile. The excellent natural harbors helped the middle colonies become traders among the colonies.
