Final answer:
The colonists lost the Battle of Bunker Hill mainly due to a lack of ammunition, forcing them to abandon their fortified positions when they could no longer repel the British forces' assaults.
Step-by-step explanation:
The main reason the colonists lost the Battle of Bunker Hill was due to their lack of ammunition. The American forces, while showing tremendous fortitude and ability to repulse the first two British assaults, eventually could not maintain their position during the third assault because they had run short on ammunition. This scarcity forced them to engage in hand-to-hand combat for which many were not equipped, as they lacked bayonets or swords. Moreover, the initial defensive success against the British also highlighted tactical mistakes on the British side, such as carrying the wrong artillery ammunition and the costly decision by British General Howe to order frontal assaults. The British eventually achieved their objective of driving the Americans out of Charlestown, but at a high casualty rate, raising American confidence despite the tactical defeat.
The organizational issues and unprofessional nature of the American force presented additional challenges, but these were secondary to the crucial lack of firepower at a critical juncture in the battle. The British would go on to win the physical ground of what is more accurately Breed's Hill, yet the American loss was instrumental in demonstrating their resilience against a superior military force and solidifying their cause. The battle had significant consequences, including the replacement of General Gage due to the public's lost confidence.