Actually it has - in combination with Neural Nets. The NN is used to evaluate the fitness of fuzzy trading rules. A company that does these kinds of things is Siemens (esp. their research department 'Corporate Technology'). Siemens uses NNs quite successfully for their own trading/hedging commodities (e.g. copper, electricity)
Supplement: As a starting point please have a look at: Siekmann, S.; Neuneier, R.; Zimmermann, H.G.; Kruse R.: Neuro Fuzzy Systems for Data Analysis, In: Computing with Words in Information / Intelligent Syst. 2; Eds.: Zadeh L. A., Kacprzyk J.;p. 35-74; Physica Verlag; 1999
Especially one of the authors, Dr. Hans-Georg Zimmermann is one of the most renowned experts in Neural Networks and Neural mathematics.