Actually saw a news report last year that broke it down. Phoenix gets way more days over 110, and the concrete just traps the heat all night. Tucson usually cools down more after sunset. That extra ten degrees on the asphalt makes a huge difference for gear and people.
My old crew in Mesa had to switch to insulated hose covers for the power stretcher. The vinyl would get soft and tacky on days over 115, and we'd get little rips. That extra overnight heat in the valley just cooks everything.