What was the mark iii incident?I'm not sure where you get that from.
Simply put a 1 series camera body takes a long time for Canon to design, develop and put into limited manufacturing, test,test,test, and then finally ready for production. After the 1D Mark III incident, I suspect Canon's gotten even more careful about pre-testing the cameras.
Then we have the fact that Canon's busy doing the 1DX Mark III - I can imagine it's easier for Canon to stagger the releases.
This really has nothing to do with the 5 series cameras.