The A321 can do LA just fine except with exceptional headwinds, and this is likely just making better use of planes that would otherwise sit on the ground longer in PHL and LAX.

US has run two eastbound redeyes before recently on peak days, such as Sunday nights, even in winter. Picking a random upcoming Sunday (Mar 27), US 1446 leaves LAX at 10:10pm and US 39 leaves at 10:30pm.