About 8 years ago airlines started charging for checked bags that they used to transport as part of a ticket.
In addition to whatever incremental revenue they may earn from doing so, they discovered a huge incentive in tax arbitrage: the more money they move out of “the ticket” (which is subject to a 7.5% excise tax) and into “ancillary revenue” (which is not subject to this tax) the better off they are financially. Roughly speaking each of the largest US airlines is pocketing $50 million a year in tax savings by charging you for checked bags.