Since the Northeastern United States contains major manufacturing areas and business centers, it's not surprising that it's the wealthiest part of the country. New York City is a major player in business and various industries. Boston is very similar in these regards. It shouldn't surprise anyone that the region is so wealthy overall.
The Northeast being the richest makes sense. This is because there are the least weather disasters and the most population there. Because of early American settlement on the East Coast that never left, and the resulting people that built companies there, the trend never really reversed itself. People who have generations of wealth from early settlers remain there too.
The Northeastern United States houses some of the nations most prestigious universities and is the birthplace of American colonization. Upon looking at the history of the United States, the Northeast generally housed the majority of aristocratic families. The Rockafeller family lived in New York and is considered one of the most powerful families in the United States since the industrial revolution. The Northeastern part of the United States is developed culturally and has prestige making it the wealthiest part of the country.
The Northeast United States region is where the English first settled, and from which the birth of this nation was launched. It is natural to follow then, that the early population centers were in the Northeast, making the area the logical place for the 'Industrial Revolution" to occur. Furthermore, it is geographically situated nearest the closest shipping lanes to Europe, our largest trading partners during that time, giving the Northeast a leg up during the earlier development of our nation. New York City, too, is the financial capital of the world, making it comparably, a very wealthy city. For these reasons alone, the fact that the US Northeast is the wealthiest area of the nation makes perfect and logical sense.