The Fox format rewards those candidates who have done the early work to gain support. They’ve already been raising substantial sums of money and gaining endorsements from influential Republicans. The candidates who are doing well in the polls now deserve to have more airtime for their hard work and early success.
Uninformed voters who may not be acquainted with the candidates need a way to separate the frontrunners from the underdogs. Cutting out low-polling candidates shows uninformed voters which candidates have a real shot at winning the nomination, and allows them to focus their attention on those candidates.
National polls this early in the process do not accurately indicate who will win the nomination, but they do show fairly well who will not.