Expensive bronze = numerically small warrior elites, supplemented by much larger number of light troops. Cheap iron = much larger numbers of well-equipped warriors. The military explanation is probably still the best.
That's not to overlook collateral effects, of course, Wider access to good weaponry implies signficant changes in social and political structure, and ultimately political power, as masses of heavy infantry trumped the palace charioteers. The emergence of the modern horse and the rise of real cavalry is also a factor.
I think Robert Drews' theory is still the best overall. He basically goes back to the change in tactics and new weaponry. Targeting the horses that drew the chariots made a huge difference in terms of changing the equation of warfare. Infantry could finally hold their own against the chariot armies. Iron was not widely used in weapons until the Iron Age II (although it is found in ever increasing quantities throughout the Iron Age I). Drews' book is called
The End of the Bronze Age: Changes in Warfare and the Catostrophe ca. 1200 B.C
Real nice post SunkenCiv and thank you as always...