Cruising is also very much a popular American vacation rather than a regular package holiday.
And most cruises are sold as a package with airfare. The changes in the how the airfare is bundled in the US would make the formation of a vacation airline even more difficult today. Back when my parents started going on cruises in the early 80's, the travel agents tended to collect all the members of the tour in one or two airports in the US and put them in large blocks of seats on legacy conventional airlines.
In '84 my parents invited my brother and myself to go on a northern European cruise with them. They had two groups flying from the US. Ours departed from JFK, the other from SEA. Travel to and from those airports was on various scheduled mainline airlines. Even as late as as 2002 my parents went on a river cruise of Russia where the bundled airfare was from JFK.
A trip they took to Italy the next year was much different. The passengers on the cruise were spread out on various airlines depending on where they originated. My parents flew to IAH on Continental where they then connected with my Dad's alumni group onto a flight on LH to FRA and later to connected on another flight to Italy. In 2006, they had similar flight arrangements to a cruise of the Danube.
In 2012, a couple of years after my father died, my Mom invited my brother and myself to go with her on a cruise of the Mediterranean through the same alumni group. They offered bundled airfare from quite a long list of cities. We opted to travel from AUS, because my brother lives in Austin not far from Austin Bergstrom. We flew AUS-EWR-BCN on United. After spending a night at a hotel in Barcelona, we boarded our cruise ship. We discovered on the cruise that members of the same group on the cruise had down vastly different itineraries to get to Barcelona. One had flown DFW-MIA-BCN. When we got to the end of the cruise in Istanbul, passengers got on all sorts flights. My Mom and brother flew back to AUS via EWR. By prearrangement, I had decided to meet do some traveling in Eastern Europe on my own eventually meeting up with my old college roommate who was living in Switzerland. The cruise travel agent made arrangements for me to fly GVA-IAD-AUS on United at the end of my trip.
The tour travel agents have the ability to put together much more customized travel itineraries using sophisticated software and databases, and the airlines are much more savvy about selling seats on their flights. As transoceanic air travel became more fragmented, the vacation oriented travel agencies followed the trend. In some cases the airline itenaries aren't even determined till just a few days before departure. The tour travel agencies sell large numbers of seats on airlines, but they are low margin. The airlines can route passengers onto fights that haven't filled up and offer good prices close to date of departure. In addition to the the fragmentation of the airline long haul market, the fact that the US population is spread out over millions of square miles means it is difficult or not optimal to try to gather all the people on a tour to one airport to use a charter or vacation airline to a destination.