DoJ apparently believes that this acquisition will remove Spirit as a competitor in the low-cost / ultra low cost airline market and thus restrain trade in interstate commerce in violation of the Sherman Antitrust Act. I do not believe that staving off a possible bankruptcy allows relief from that.
It's not like JetBlue is swimming in money either. In 2022 JetBlue reported losses of $280 million on revenues of $9,150 million. Spirit reported losses for 2022 of $60 million on revenues of $5,060 million.