As one who flew ORD-SFO somewhat regularly for work in the mid-2010s and a little bit during the COVID era until a job change, I’ll note that widebodies haven’t all that common other than maybe 1-2 flights/day. When I was traveling the most, it was typically a bunch of 753s and 739s, with an HD777, the occasional A320 (which seemed like a fleet positioning move more than anything), etc. International WBs were a treat. After the PW777 grounding, the route got more international 777s and 787s, but once the fix to the PW fleet was certified, those Polaris-configured planes were dropped in a hurry (much to my chagrin, as one who had a premium economy seat assignment on an international config scheduled for just a couple days after they were allowed back in service, only to have every WB on the route swapped back to an HD almost immediately).

As others have noted, getting a Polaris WB on a domestic route like that is limited to off peak, fleet positioning, or specific route demand reasons (i.e., EWR-SFO/LAX). During the summer, not only are the Polaris jets used for far more lucrative international travel, but UA at ORD is limited in the number of gates that can handle 777s/787s, so if they are flying more international service, not only are they using up the planes, they are also using up the gates.

All that to say, the lack of “nice” widebody planes on ORD-SFO is nothing new. If anything, regularly scheduled Polaris-equipped planes on that route is more of an anomaly than a standard.