In recent years, credit card companies have realized that in the perennial competition to acquire and retain customers, they need desirable rewards partners. While many card issuers have their own rewards points or cash-back programs, many consumers want to earn currency from a particular brand.
Airlines are usually near the top of the list in terms of desirability. They serve huge numbers of customers. Moreover, airline tickets are expensive enough that plenty of people want to earn points for free flights.