The ability to upgrade on particular routes has been dramatically affected by the availability of attractive YUP fares. I don't know if it's still the case, but for a long time, the NYC-LAX/SFO flights were among those routes. And the fact that the flights had plenty of availability in F (or J) a few days before departure doesn't really change the analysis: since people purchasing last minute tickets tend to pay the highest prices, many have figured out that they can get a YUP fare (and a first class seat) for no or little additional charge.