The reason why UA has better FC service LAX-JFK-LAX on their 3-class planes is because they, as does AA, tout there "premium trans-con service". There's a highly outdated explanation of this on UA's websight. AA actually refers to their FC on these routes as "Premium First Class".
UA and AA are catering to Hollywood, note Billy from ALLY MCBEAL on your flight. The majority of those in F and C on both UA and AA have paid the full fare, which can run in the neighborhood of $3000 to $4000 round trip for C to F service.
On AA it is very difficult to get an upgrade from Y to C unless you're a high-level elite. Dropped a friend off at LAX awhile back. He's gold on AA, something like 27 people on the waiting list to get into C.
All of the movie/tv studios, agencies, etc. have corporate deals with either AA or UA and there is a tremendous amount of business done between NY and LA, thus being at the first level of elite on either carrier for this route is practically useless. I believe Universal Studios has a deal with UA and Creative Artist Agency has a deal with AA, etc.
That said, I take DL as I have no problems ever getting upgrades on their flights because their sched between LAX and JFK is designed around their flights coming and going from Europe. And now that the 767-300ER's are equipped with the Business Elite seats and video monitors, the 5 to 6 hour flight can be quite a nice ride. ***I just did LAX-JFK yesterday on DL and will post the trip report later today or tomorrow.
Unless one lives near EWR, there's no reason to fly two-class narrow body service from LAX.
the scribbler