The prices in the graphs goes back one year only but from my own data collecting this trend goes back further than that.
Theories and my thoughts about them:
One theory is the lack of major wars. But by the looks of it lack of wars is not the reason.
The new ships that were introduced to the game in June last year. people are switching to the new ships that dont use salvage.
More players in EvE, people join, salvage, thus the price goes down.
CCP has changed the drop rates.
I want to know what you think is the reason for the price drop.