We used to have 24F seats on the 757s at US prior to the AWA merger ourselves - and they were always full. But then Tempe started using the East 757s exclusively for low yield, high capacity runs and wanted to cram more seats into coach and make it a cattle car (which they certainly succeeded in doing), so FC was practically removed for the aircraft altogether. Not saying I agree with that decision, but that's Tempe's explanation for it.
Wait 'till Dougie becomes the CEO of UAL...................