The plane was at 37,000 feet, cruise altitude, flying to PHX, all the other planes in the area were either on final, or on climb out, so they would not be making contrails like US808 was that day, and the next day. Very simple answer to your question.
There are a couple things that confuse me about the airplane theory.
First of all, if you look at a map and draw a direct line between Honolulu and Phoenix, the first place that line touches the North American continent is the very northern part of Baja California. Assuming that US Airways would want to stay out of Mexican airspace, it seems more logical that the flight plan would take the plane over San Diego, rather than Los Angeles. Not only would Los Angeles be out of the way because it is latitudinally north of Phoenix, but Los Angeles has one of the busiest airspaces in the world. Why would a plane that wasn't going to land in LA fly over LA, especially if it would add to its' flight time and distance? And, while I realize the US Airways route map is probably not the most accurate depiction of their flight plans, it does show their non-stop flights from Honolulu to Phoenix passing over San Diego, not Los Angeles. Has anyone from US Airways confirmed that Flt 808 flies over LA?
Second, the photographer who filmed the footage said the object flew off towards the northwest. That would mean it flew away from the city, not over it, and certainly not to Arizona.
Third, there has been a lot of speculation that the verticle flight of the object was an optical illusion because the object was actually flying towards the camera. Yet that theory contradicts the photographer's report that the object flew northwest. In addition, if the object was flying from west to east (towards the photographer), we should have seen two contrails. I've never seen a commercial aircraft fly overhead with only one contrail.
Bottom line, I don't believe for a second it was US Airways Flt 808.