I realize that unless a US insider posts with hard data, we are all just speculating. And that's really all I was asking for. Just some speculations as to why this might be the case.
I didn't answer my own question and as SS255 said, if the 5:50 doesn't work for me, the 7:30 probably isn't going to work either by the time you factor in the additional drive time (or at that time of day, the additional sit and wait in traffic time).
As recently as last month I flew back on a late flight from LAX. I think it was at 9:30 and it was full.
US isn't even selling a code share on UA after 5:50.
LGB and BUR use RJ's for the later flights and there are flights in the 7 o'clock hour from ONT, SNA, and SAN. So every major airport in the SoCal area except THE major airport has a later flight. Not that it couldn't be the case but I'd be surprised if there just isn't demand for a later flight out of LAX.
Don't get me wrong. I'm not saying that US is wrong and doesn't know what they are doing. I just find it interesting that the last flight is not only relatively early but at an odd time too. At least if it was 6:50ish people could work most of the day and still make that.