The British army had 50,000 well trained and armed troops. They had strength by numbers since the colonists didn't have as many soldiers. When comparing weapons the guns that the colonists had were like "toys."
Distance from Britain to America was far
The British had money for the weapons and proper supplies for the troops. Since they were taxing citizens of Britain it was possible to receive enough money. King George supplied the troops with 30,000 mercenaries.
The soldiers lacked patriotism
Britain was able to lure away many of the African Americans to join their army. They promised freedom and land in their country if they fought for them. This caused the colonist's army to weaken because their army lacked solidiers on certain occasions.
Generals had problems with strategic location
The distance between America and Britain caused problems with shipment of items. Sending the troops supplies was a problem since it traveled by boat across the Atlantic ocean. The news of battles reached Britain long time after the event occured which caused problems with planning.
When first planning war against the Americans the citizens of Britain believed it was a lost cause. They didn't think it was worth risking lives and wasting money on this war. The Americans had an advantage to that since they were fighting for their freedom.
The soldiers that never went to the colonies were unfamiliar with the layout of the land. Things like hills, valleys and coves in the American land were like foreign obstacles. They were fighting in their enemies land and the Americans took advantage of that weakness.