Opportunity costs is the cost of the next best alternative use of money, time, or resources when one choice is made rather than another

Opportunity cost:

When an individual at a given instance and for a given chance chooses one alternative over the other, there are chances that he may lose the value of the former. This concept is referred to as 'opportunity cost' in short. It is a theory between the scarcity and choice of the consumer.

It is simply because choosing an alternative over the other costs the individual the 'opportunity' to choose the other alternative.