High-strength aluminum alloy is a preferred material in the manufacture of stair stretchers due to several reasons. Firstly, it offers excellent structural integrity and resilience, making the stretcher capable of safely carrying heavy weights. Secondly, it's relatively lightweight compared to many other industrial-strength materials, which makes the stretchers easier to handle and maneuver. Lastly, it has excellent resistance to corrosion, making it suitable for use in varied environments, from outdoor emergency evacuations to indoor settings like hospitals.