America West started with the concept of dual hubs in PHX and LAS. The goal was to increase aircraft utilization by operating a normal, daytime hub in PHX. Then, rather than sitting on the ground all night long, the early evening arrival in the spoke city turns to a "night flight" to LAS (which connected to one of two banks of nighttime flights at LAS).

When fuel was cheaper and fares (inflation-adjusted) were higher about 10 years ago, this concept worked well. The LAS traffic just had to cover the marginal cost of operating the flight (fuel plus staffing) because the aircraft was already paid for by the daytime flights.

At $135 fuel, it just doesn't make good business sense anymore. If fuel goes back down, you may see a re-opened nighttime hub at LAS for US.