The collapse of Spirit Airlines isn't the only curveball confronting people planning summer trips. Rising jet fuel costs tied to the Iran war have pushed up airfares and associated fees across the industry. Two of the remaining U.S. budget carriers just finalized a merger. The developments illustrate how difficult it's gotten for low-cost airlines to operate while squeezed by jet fuel prices, changing consumer preferences and competition. For decades, budget carriers thrived by targeting price-conscious passengers. But big airlines like American, Delta and United have gotten better at tailoring prices to different travelers and matching low fares. Major carriers also can more easily generate revenue to offset higher fuel costs.