There were probably many passengers on UA2423 connecting to long haul destinations in the 11pm-midnight bank (SYD/MEL/BNE/AKL/SIN/TPE/MNL/HKG) such that UA would want to prioritize the slot to ensure these passengers make it to their connecting flights, vs UA1230, mostly passengers destined for SFO or connecting to other cities on the west coast that can be more easily accommodated.