In reality, there were huge numbers of colonists who didn't support the American Revolution. After the initial fervor died down, just getting people to enlist proved tough. The patriots had to offer large bounties to get people to sign up for the war, and when that didn't work, most colonies resorted to conscripting men, sending the conflicting message, "Cast off the chains of oppression or we'll throw your ass in jail."
On top of Americans being far more divided than the movies would have you believe, even historians forget about the opinions of a huge population, one that the fledgling U.S. government also conveniently overlooked: the slaves. It's easy to see why many American slaves would have been on the side of the British: They didn't like being fucking slaves. And as a slave, that's probably your number one political issue.
Slavery was already outlawed in the United Kingdom and its island territories, which meant that it would likely be on its way out in the Colonies if the British prevailed. In fact, Lord Dunmore, the royal governor of Virginia during the revolution, announced that any slave who joined the British against the rebellion would be granted freedom. "Tyranny" under the British monarchy meant freedom for the slaves.
As a result, many slaves fled -- or attempted to flee -- including Harry Washington (three guesses as to who his master was). Henry Washington joined the British army in Lord Dunmore's regiment, who wore uniforms embroidered with "Liberty to Slaves." Unfortunately for Henry and thousands of other slaves on the run, the British surrendered to the colonists fighting for "freedom" -- including the freedom to keep slaves. Many former slaves scrambled to make it aboard retreating British ships. Female slaves who were pregnant were especially incentivized to make it across British lines, because any baby born there would receive a "BB" certificate (which stood for "Born Free Behind British Lines").