Ultimately it’s because it works well. Using your left hand, it allows your thumb to operate the space by bar, keeps the number keys (typically used for weapons) within easy reach, and allows the easy to hit tab, shift, alt and Ctrl buttons to be assigned to additional operations with the little finger.
Early pro-gamer Thresh made the configuration popular. A lot of gamers copied this because they wanted to be like him, and it became pretty popular.
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