My guess is because UA figures the plane sits in HKG overnight unlike many of the other 744 which turn right away. The only impact a delay has for SFO-HKG is misconnecting passengers in Hong Kong or crew legalities for the return trip 2 days later.

Earlier this morning a friend messaged me about the impending delay for UA 869 because they app was showing the inbound (N128UA) as UA 769 ORD-SFO which at the time was suppose to arrive at 1309. N128UA now heading to HNL on UA 300 departing at 1315.

Looks like there is a broken 744 in PEK (N107UA) so they had to wait for same day arrival to run UA 889 (N174UA) on 12/26 which they are turning to UA 869 SFO-HKG. Will be interesting to see if there are any issue on 12/27 with UA 889 PEK-SFO as they had to use the frame one day early to operate UA 889 on 12/26 since N107UA appears to be broken, hence the 5 hour 28 minute delay for UA 889 on 12/26.