Probably because there's no reason for United to deeply discount the EWR-SFO premium cabin to the general public all day, every day. The corresponding fare exists but they just don't put too many in inventory. It's the highest grossing single-airline/single-route combo in the USA by a margin, and that's attributable mostly to the big seats.

I know there are a number of major corporate clients who have preferential volume-based discounts on EWR-SFO that can be in the ballpark of P fares under certain conditions, but those generally book into J/C/D or even Z, which is more prevalent.