Because any subsonic will generally take more or less the same amount of time to cover that distance, regardless of the size or the seating arrangement. Sure, there are differences, but not enough to make it an issue. I've flown the American Eagle shuttle between DCA and LGA on an Embraer (1-2 arrangement) and it takes the same amount of time as the US Air shuttle with the 3-3 arrangement. (I prefer to take neither and to use the Acela so as to eliminate the need to get from the airport into Midtown, but on weekends in particular the Acela schedule doesn't always work.) Concorde, on the other hand, took me from LHR to JFK (3,458 miles) in three hours 21 minutes, and no other aircraft available to me could have done that. There's the difference. If I really, truly care about the seating on a short-haul flight, I can usually (not always, but usually) find a variety of options that take about the same amount of time. (The DCA to MLB route can only be done nonstop on a Delta CRJ, for example, whereas DCA to BOS or EWR has several options ranging from Embraers on up.) But there was only one way to make that supersonic trip.

Just for the sake of comparison, I flew in Upper Class on VS from LHR to IAD this past Sunday. The seat was very comfortable and the flight took 7 hours 10 minutes, roughly. We left the ground at LHR at around 18:00 and landed at IAD at around 20:10 or so; I got home at around 21:30 after clearing immigration and retrieving my car. Had that been Concorde, the flight would have landed at around 17:00 (fully three hours 10 minutes earlier local time) and I would have been home substantially earlier. You can pooh-pooh the time issue all you want, but some of us would rather have that benefit.

Washington to Dallas doesn't realize the same benefit because Concorde had to fly subsonically on that route. That's the major weakness of SST designs to date. Nobody has ever contended that Concorde, or any other SST, should replace ALL subsonic routes. There are places where it would make absolutely no sense, such as that DCA to LGA shuttle route (too short a flight for it to make a difference)....or consider the Loganair flight from Westray to Papa Westray. You appear to be making the assumption that I'm saying that Concorde is always better, and that wasn't the point at all. As for that route from LHR to Singapore, I'd still rather take Concorde and save the five hours. Time is valuable.