Like all the other SYD flights, I assume quite a few people will be connecting LAX-SYD-MEL with AA then QF.
MEL's problem is it is not well located for connections to/from the US, extra hour to/from most spots. And Sydney has more, what I would call "classic" or "obvious" touristy things to do (Harbour Bridge, Opera House, etc.) than Melbourne does. Melbourne has plenty as well, they simply aren't as well known here in the US. I know plenty of great things to do in and around Melbourne, and as much as I personally prefer Melbourne to Sydney any AND EVERY day of the week
, the awareness here in the states of Australia is much more of Sydney than Melbourne. Combined with SYD having a better location for most transfers (particularly to/from NZ when traveling to/from the US, and CNS/Great Barrier Reef) than MEL, and both having roughly the same amount of business traffic (I believe that is the case but don't have solid numbers to back that up), and SYD simply having more destinations served within Australia than MEL does, it is pretty clear to me why SYD sees far more USA flights than MEL does.