Supply vs Demand
7D is 4 years old. The OMG Its a new camera I must have it phase is over with. Furthermore, when you buy an item, youre not only buying the physical item, youre buying the R&D that goes into that item. Being 4 years old, the R&D has likely been paid off.
5DIII is now has a supply somewhat equal to its demand so prices are stabilizing.
The 5DII now has a supply higher than its demand due to the 6D and 5DIII, so its price will continue to slip until its discontinued.
And so on and so forth. Not many things stay at the same MSRP for its entire sale life.