The delegates to the second continental congress chose george washington as commander in chief because

Reasons for choosing George Washington as Commander in Chief:

They wanted a commander in chief who had solid combat experience, and George Washington had a strong record from the French and Indian War.George Washington, at age 43, was both old enough to be respected and young enough to have the vigor and stamina for the battle campaigns that lay ahead.Among the colonies, Virginia had the most wealth and population. having a commander in chief from Virginia made sense as Virginia was key to the success of the overall effort.Virginians at the Second Continental Congress convinced congress delegates from other states of Washington's character and determination as a patriot.

For more details on George Washington's appointment as Commander in Chief, you might want to look at materials available from George Washington's Mount Vernon, the museum of George Washington's estate in Virginia.