Final answer:
The Green Mountain Boys were able to take the fort easily due to the element of surprise, familiarity with the terrain, and the British troops' exposure. The militia and minutemen were able to take shots at the British soldiers from advantageous positions, while the British soldiers had no cover from enemy fire.
Step-by-step explanation:
The reason it was easy for the Green Mountain Boys to take the fort was because they had the element of surprise and the advantage of familiarity with the terrain. The British troops, under Colonel Smith, were marching along a narrow road, exposed and unable to hide from the colonists who were able to take shots at them from advantageous positions. The militia and minutemen could retreat into the shelter of the woods and move down the road to find new positions from which to attack, while the British soldiers had no cover from enemy fire. The British troops were outnumbered and eventually turned back, giving the Green Mountain Boys the opportunity to take the fort.