There are three main reasons for the diversity in the Middle Colonies as compared with the New England and Southern Colonies. The first and possibly the most important is the geography of the colonies. The Middle colonies benefited from having a very diverse geography, which lent itself to more economic opportunity than the other colonies. The Middle Colonies became known as the "breadbasket" for the early colonies. The soil was fertile, not rocky, and the hills were gently sloping, making the region perfect for certain types of critical agriculture. The climate is moderate with a long growing season and short winter. Grains like wheat and grazing animals like cattle were significant agricultural products produced. However, this was not the only geographic advantage.

The Middle Colonies had many places where open access to deep water and protected coves line the shoreline. Deep water allowed for numerous entries into the continent from the ports, and because of the milder climate, they were able to attract numerous immigrants from all parts of Europe.

The second reason is that the Middle Colonies were diverse in religious practices. There was a tendency toward being much more liberal and open regarding religious practice. This was probably the result of the diversity of immigrants attracted to the colonies and settled by Europeans.

The third reason is harder to document with hard evidence, but anecdotal evidence suggests the Middle colonies were interested in making sure there was at least a rudimentary system of education available to citizens. There were many efforts to develop a public education system. A strong foundation of education tends to result in people thinking differently about the world around them and tends to attract diversity. Progressive, tolerant attitudes are the result of education.

Why were the middle colonies such diverse colonies?  

The middle colonies—Pennsylvania, New York, New Jersey, and Delaware—were diverse for several reasons. First, they were initially settled by groups of different nationalities. For example, the English settled Pennsylvania, while New York, New Jersey, and Delaware were initially settled by the Dutch. Each group brought with it its distinct language and culture. The strong Dutch presence is a main difference that marks these colonies from the overwhelmingly British settlements to the north and the south.

Beyond that, these colonies opened themselves, whether willingly or reluctantly, to groups of different nationalities and ethnicities. For example, William Penn, a Quaker, started Pennsylvania as a colony based on religious freedom. Sensitive to the persecution that the Quakers had suffered in England, he wanted to open the new territory to other persecuted groups. He invited in persecuted Anabaptists from German speaking states and Switzerland, bringing in a culture that became known as the Pennsylvania Dutch. Penn also promoted peaceful relationships with the native groups in the area.

In addition, more than merely the Dutch colonized Dutch holdings. Swedes and Finns, for example, also settled in New Jersey. Further, while the Dutch at first tried, unlike William Penn, to keep all but people of the Dutch Reformed church from New York (at the time called New Amsterdam), people of other religions came and stayed. For example, a group of Jewish refugees arrived and settled permanently in the colony in 1654.

It should also be noted that Native American groups were a strong part of the cultural mix in all these colonies, especially in the early years of colonization. Although, as Charles Mann argues in his book 1491, many natives had already been wiped out by European diseases brought by traders and missionaries by the time colonies were established, many natives still survived and interacted robustly with the Europeans.