I can only assume that coffee, like everything else, has gotten more expensive to grow, harvest, process and transport.
I buy my coffee green and roast it myself so there's very little value added to it in the US. When I first started home roasting in 2005 I could usually get beans from my supplier for $3.??. Now the bargain beans are $5.??. I doubt that the cost of growing inputs in east Africa or my supplier's overhead have changed very much. The only thing that has changed is the transport costs to get them from Addis or Nairobi to the port of Oakland.