It was an engineering requirement. The Shuttle is essentially a "Single Stage To Orbit" booster. Kerosene/LOX works well for the first stage of a booster in the lower atmosphere (i.e. Apollo), but it doesn't work well in higher atmosphere or Space. That's why they HAD to use Hydrogen/Oxygen for the Shuttle Main Engines.
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It was an engineering requirement. The Shuttle is essentially a "Single Stage To Orbit" booster. Kerosene/LOX works well for the first stage of a booster in the lower atmosphere (i.e. Apollo), but it doesn't work well in higher atmosphere or Space. That's why they HAD to use Hydrogen/Oxygen for the Shuttle Main Engines.