Final answer:
The British attack wasn't a surprise to the colonists because they had received alarming information about the British intentions and key leaders had already left Boston. The militia stores in Concord had also been moved to other towns.
Step-by-step explanation:
The main reason the British attack on the night of April 18, 1775, wasn't a surprise to the colonists was because the colonists had received alarming information concerning British intentions. They knew that General Thomas Gage was ordered to seize the munitions and leaders of the rebellion and had taken action to locate them. Additionally, key leaders like Samuel Adams and John Hancock had already left Boston for Lexington, and the militia stores in Concord had been moved to other towns in advance.