Delta made a recent change (~2/14) that treats BE seats on JFK-LAX/SFO/SEA like transat/pac routes. This excludes these buckets from cheap, day-of upgrades and complimentary upgrades for FFs. The assumed intent is to generate more revenue on their highest demand routes bw the most affluent US economies.

If you believe there's a reasonable chance you'll get F for $250, instead of $1750 more - you'll go for the $250 bet almost every time. If you believe there's no chance, you're more likely to spring for F on a special occasion, red-eye, etc.

If we apply this to the 15% of passengers on this route like you (~30pax on a 763), and say 10 of them hold out for, and then pay $250 for the day-of upgrade, addl rev for DL would be $2500. Now if those same pax know the $250 upgrade it isn't possible, and only 2 of the 10 pay $2500 for F, addl rev for DL would be $3500.