Why were alkaline fuel cells used on U.S. space shuttle orbiters?

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Diagram of a proton exchange membrane (PEM) fuel cell showing labeled layers, including flow field plates, electrodes, and the proton exchange membrane, with arrows indicating hydrogen and oxygen inputs, electricity generation, and water and heat as outputs. — fuel cell Proton exchange membrane (PEM) fuel cell
The proton exchange membrane is one of the most advanced fuel cell designs. Hydrogen gas under pressure is forced through a catalyst, typically made of platinum, on the anode (negative) side of the fuel cell. At this catalyst, electrons are stripped from the hydrogen atoms and carried by an external electric circuit to the cathode (positive) side. The positively charged hydrogen ions (protons) then pass through the proton exchange membrane to the catalyst on the cathode side, where they react with oxygen and the electrons from the electric circuit to form water vapour (H₂O) and heat. The electric circuit is used to do work, such as power a motor.

Alkaline fuel cells were utilized on U.S. space shuttle orbiters because they provided a reliable and efficient power source for manned spacecraft.12

While the design of these cells presented significant engineering challenges—specifically because the strong, hot alkaline electrolyte tends to penetrate structural joints and attack many plastics—these issues were successfully overcome to enable their use in space flight. Like other hydrogen-oxygen fuel cells used in earlier missions such as Apollo and Gemini, these systems offered the distinct advantage of producing a vital byproduct: drinkable fresh water. By continuously combining hydrogen and oxygen, these fuel cells were able to deliver an unwavering supply of electricity throughout the mission.13