It’s the biggest tabloid tale since O.J. Simpson, but so far the networks are not scrambling to make a movie about the alleged murder of two little boys by their mother, Susan V. Smith, in Union, S.C. That’s because ”ripped-from-the-headlines” TV movies of the week just aren’t drawing big audiences anymore, says industry R&D’s Tom Colbert, who tracks true-crime stories for Hollywood producers. ”The public just wants to be entertained at night,” says Colbert. Indeed, ABC declined to go forward with its much-hyped Nancy Kerrigan-Tonya Harding project. In fact, according to TV sources, several producers did approach the networks about a Smith movie, but executives rejected it as too widely hyped by the media — and too downbeat. Maybe there is some accounting for taste.