Mere hours after Spirit Airlines shut down this month, JetBlue Airways announced more than two dozen new flights out of Spirit’s home airport. JetBlue’s speed in filling the void was not surprising. The airline is trying to turn itself around after years of losing money. JetBlue wants to make Spirit’s base — Fort Lauderdale-Hollywood International Airport near Miami — the next pillar of its business, and seized the opportunity to attract thousands of travelers suddenly left in the lurch. Once celebrated as a hip, disruptive upstart, JetBlue has fallen on hard times. The company has shrunk somewhat in recent years to under 5% of domestic air travel, making it about one-fourth as big as the country’s largest airlines. As a result, it is not big enough to compete head to head with American, Delta, United or Southwest. And its costs are too high to pursue the low-fare approach popularized by Spirit and others. That leaves JetBlue in a difficult situation: It needs to grow, but it has few easy or good options.…