Conventional wisdom says Demi Moore's The Scarlet Letter failed at the box office because it's a strange adaptation of a literary classic. But we have another theory: The key to Demi's success is the length of her locks. Look at the facts: She wore a stylish pixie in Ghost (1990), and the grosses went out of this world; she flaunted messy long tresses in Scarlet, and Letter was canceled at the box office. ''I don't buy it,'' says a spokesman for the actress. ''Moviegoers love Demi Moore whether her hair is long or short.'' Still, perhaps Moore should take a look at the evidence before deciding on a coif for the upcoming Striptease.