One of the original stipulations of the legislation was to give carriers access to DCA who did not have hubs inside the perimeter. Which is why in the original allocation Frontier and the old America West won most of the slots (5/6 IIRC) with TWA winning a single LAX daily. When TWA was taken over by AA the slot went back in the pool and was reallocated to Alaska (who did not apply in the first round) for DCA-SEA just prior to 9/11. So the original 6 slots ended up in the hands of carriers who basically had no presence inside the 1250 perimeter (HP had a small hub in CMH that has been closed down). In the next round of allocations AS won 2 more (SEA & LAX), HP won 1 (PHX), F9 won 1 (DEN), DL won 1 (SLC) and UA won 1 (DEN). The justification for DL and UA was to connect smaller communities in the Rockies to DCA with a single stop. To my knowledge the only carrier that has ever applied for DCA-SFO was US who was promptly shot down for being the majority slot holder at DCA. When HP took over US their slots did not go back up for reallocation, which IMHO was a mistake on the DOT's part. Now the combined carrier has the most slots at DCA and the most beyond-perimeter slots.

(That is all from memory so pieces may be wrong, please correct me if it is).

ETA: Lots is wrong so see post below.