There were two main reasons why the Second Continental Congress chose George Washington (rather than some other person) as the commander in chief of the Continental Army. One reason was political, the other was military.
On the military side, Washington was selected because of his experience. Many colonists had been part of the British forces that fought in the French and Indian War. Of those colonists, Washington was seen as the man who had been most involved in the war at the highest levels. Therefore, he was seen as more qualified militarily than the more political leaders of the revolution such as John Hancock.
On the political side, we must remember that the colonies did not have a strong sense of unity at this time. People thought of themselves as citizens of their colony, not as Americans more generally. For this reason, it was important to make efforts to ensure that all colonies would be behind the war effort. Most of the political leadership of the rebellion was from Massachusetts. Therefore, the Congress chose Washington as commander in chief partly because he was from Virginia. This selection would help to ensure that Southerners would support the revolution as well.