Monopolistic firms tend to earn an economic profit.
However, it is possible for every firm in competitive markets to earn economic profits too! For example, imagine a competitive market for snow shovels, and a freak snow storm drives up the demand for snow shovels extremely high. Everyone will be able to make a profit until more suppliers join the market.
Given that you can make a profit in either kind of market, why might economists be more concerned about the profits they see in monopolistic markets than in competitive markets?