The answer is: luggage

According to today's USA Today, "Few get to airport by bus, train." Since I don't have a car, I get to and from airports mostly by public transit, and occasionally by taxi, depending on the time of day (early or late flight, before or after the subway runs).

I have noticed that, except for WMATA's B30, most public transit buses* that provide service to airports don't usually provide additional accommodations for luggage. (I can't remember if the WMATA bus that goes to Dulles has has luggage racks; it's been a few years since I've ridden it.)

While subway cars don't provide luggage racks either, it usually is easier maneuvering luggage around a subway car than a bus.

In any case, I have suggested to people getting out a Union Station to take taxis rather than continue via bus-based transit to get to their hotels.

(* I can think of buses in Seattle, Pittsburgh, and Minneapolis providing service between the airport and the city that don't have luggage racks. If you carry more than one bag, it's a real pain. And it can be plenty inconvenient even with one bag. Or if the weather is nasty, i.e., cold rain in Seattle.)