It's a bit anachronistic to think of British North America as "divided" into 13 Colonies, as if it was one unit of territory divided thirteen ways at once by conscious decision.

In fact, the colonies in British North America were founded as independent ventures over a long period of time. Virginia was founded in 1607, and Georgia was founded in 1733, a mere 42 years before the Revolution. And this is just the span of the "13 Colonies", as opposed to all of the colonies in British North America: Newfoundland had its first English fishing posts established at the end of the 16th century, and the colonies of East and West Florida were established in 1763.

"Colonies" also includes a wide range of entities. The original colonies established by the Virginia and Plymouth Companies were similar to contemporaneous efforts by such outfits as the East India Company and its non-English equivalents in Asia: private companies were granted government charters, effectively legal monopolies, to engage in certain kinds of trade in "factories" for profit. Various other companies would be granted charters for founding colonies over the course of the 17th century, and these were increasingly for reasons other than trade and economic gain: the Massachusetts Bay Colony, for example, was founded to provide a place where Puritan English Protestants could settle and create communities according to their own values (many of the other New England colonies were founded by other Protestant groups escaping Puritan control, for example the Baptist Roger Williams in Rhode Island).

In the case of Virginia, this failed completely, and in 1624 Virginia was turned into a "crown colony", ie a type of colony that had a royal governor appointed by the government who oversaw governmental functions in the colony. Over the course of the 17th century is became the preferred type of colony, and most charter colonies had their charters revoked by the Crown and became crown colonies. In the case of New England, New York and New Jersey this went hand in hand with creating a "super-colony" under James II called the Dominion of New England, but this lasted a couple of years until it was abolished during the Glorious Revolution.

While crown colonies eventually became the dominant form of colony in the Thirteen Colonies, even at the time of the Revolution there were still so-called Proprietary Colonies in Pennsylvania, Delaware and Maryland: these colonies operated under royal charters granted to Lords Proprietors (members of the Calvert family for Maryland, and the Penn family for Pennsylvania and Delaware), who ultimately had the final say in terms of how the colonies were governed.

So in addition to the Thirteen Colonies being founded at different times by different groups for different purposes under different forms of government, why was it hard to unify them? In simple terms, because of the exceedingly slow pace of communication and transportation. Travel between colonies was slow, arduous, and expensive, whether by sea or by land. Distances for travel and communication were of such a concern that it actually led to colonies breaking up rather than unifying: in 1712 Carolina split into a North and South in no small part because the settlements around New Bern (in North Carolina) felt that they were too distant to be effectively governed by the colony based in Charleston (South Carolina).