Summary: Once a large hub for US Airways, the airline has made sweeping cuts there in past years. The Las Vegas Sun writes the carrier's "latest cuts will reduce US Airways' Las Vegas flight schedule from 64 a day to 36 by the end of February. When operating as America West Airlines in the 1990s, the company – (currently) the second-busiest operator at McCarran – had around 140 daily flights."
US Airways is probably balancing traffic in Phoenix and Las Vegas using some undocumented, or at least undisclosed, formula that might or might not have any bearing on reality.
The sad truth is the US Airways has made a series of poor decisions since the merger with, which is better characterized as a hostile takeover by, America West.
Moving customer support from North Carolina to Arizona was the first mistake, and the initial data point in my decision to leave US Airways as a frequent flyer.