Why did adam smith insist that a market economy (capitalism) would produce consumer goods more efficiently than a command economy?

The "invisible hand" is the interaction in markets where those who are able to provide goods or services find agents who demand those those goods or services and are willing to exchange them for something the supplier deems of value. If the demand is met, or there is demand by another agent in the market willing to provide compensation to the supplier that is deemed of more value, the resources in the market look to supply the next need for valued consideration. 

In a command economy, supply may not be demand driven. This may result in excess capacity devoted to something that does not allow the highest value to be placed on the supplied goods or services. A transactional inefficiency may then be the result of a command encominy that is not focused on markets