From this website:
Enter Batwoman
This character made her debut in Detective Comics #233 (July 1956), written by Edmond Hamilton and illustrated by Sheldon Moldoff. It seems clear that her introduction was part of a publishing decision to alter the ambiance of the series. Memories are vague on this point, but the responsibility may have rested with DC publisher Harry Donenfeld's son Irwin. After Whitney Ellsworth moved to California to produce a Superman television series, Irwin Donenfeld was serving as de facto editor in chief. "He came in and he had a lot of things that he sort of threw at us," recalled Batman editor Jack Schiff. Then again, Schiff himself believed new characters would bump up sales, and everyone seemed eager to provide Batman with some female companionship.