The digic chip in the camera works with several other microprocessors, and the sensor has a increased number of readout channels requiring more pins on it and on the motherboard. That's needed to get faster readout Its not something easy to try to shoehorn updated technology into a old design. Even the motherboard would not fit into a 5D. And, who would want to spend $XXXXX upgrading a 5D I with custom motherboards and customized microprocessors when only a few hundred might sell?. Its a losing situation.it requires to change the processor too because IMO sensor + processor = film replacement - but that doesn't drive the price too much.
Ugh, why's that? Surely a processor can be designed to work with different sensors?
Ok, imagine that the Canon 5D had a replaceable sensor.
Now put the 5D Mark III sensor in the 5D and keep the 5D processor. How many fps are you going to get?
But there is nothing to say that Canon couldn't supply a new DIGIC chip with the same piece of hardware that supplies the new sensor.
Its like trying to put a modern computer controlled car engine that interfaces with several computers in order to work into a older car that either did not use computers at all, or had much simpler ones that had fewer wires and simple but incompatible functions. It can be done, but the cost is more than a new car.