To anticipate the next topic..."I want [insert airline here] to fly to [insert destination beyond 1,250 miles from DCA here]"
In the byzantine set of congressional/DOT regulations associated with the Perimeter Rule (which restricts flights to/from DCA to 1,250 miles) there are 4 "wild card" slots that were awarded to the 4 major carriers in 2012:
US: SAN (changed to LAX after the AA merger)
AA: LAX
UA: SFO
DL: SLC (this is the one being changed to LAX)
These 4 slots can be changed anytime to any place. All other flights (including those already operating beyond perimeter) require government approval to change.
DL already had a SLC slot from prior congressional/DOT allocations. To start DCA-LAX they
gave up one of the 2 SLC slots reluctantly and moved the service to IAD.