But Why Not (foaled in 1944) was an American Thoroughbred racehorse. The daughter of Blue Larkspur and granddaughter of Black Servant is probably best remembered for wins in the Alabama Stakes, the Acorn Stakes, the Black-Eyed Susan Stakes at Pimlico Race Course. In 1947, she was voted by the country's top sports writers as the American Champion Three-Year-Old Filly.