Read this interview. Courtney Solomon didn't want to direct that D&D movie, just produce it. He brought James Cameron, Francis Ford Coppola, Renny Harlin - and the idiot boss at TSR rejected them all. Eventually, Wizards of the Coast bought TSR, and the next idiot in charge promptly sued Solomon. They settle, but part of the settlement is that the movie has to begin production real quick - ruining their shot at finding investors for a bigger budget - and use the same script TSR had approved years before - even though they had a much better one ready.
To sum it up, the idiots who owned the D&D brand forced the movie to fail.