Frequent travelers typically are members of one or two frequent flyer clubs. They try to minimize the number of airlines that they use so as to easily and quickly accrue large amounts of frequent flier miles on the one or two airlines. By default, then the frequent flier miles quickly accumulate in one frequent flyer program therefore enabling the frequent flyer to have free flights sooner.
Airports typically have one or two airlines that are the largest or that have the most flights that arrive or depart from that airport.
A frequent flier that travels for business will typically use those airlines because they provide the most options to fit the schedule they need to either fly out or fly home (for example in Dallas, American Airlines would be the choice at the DFW International Airport, while in Atlanta it would be Delta). Since the business frequent flier will be able to get reimbursed for the cost of their flight from their employer, the price of the flight, while important, will many times be a secondary concern. Most companies will have a travel policy that will provide guidelines on the costs that an employee (business frequent flier) can pay for a flight.
Additionally, the frequent flier that travels for leisure will try to pay for their flight using their frequent flier miles that they have earned/accrued. If they do not have enough to exchange (a domestic flight is will typically cost 25,000 miles from the freq flier account) then a leisure frequent flier will probably try to balance a low price flight with the schedule that they are trying to achieve.
