What in terms of the physical makeup of the sensor does Sony have going for it, and what was, or is preventing Canon from doing the same thing?
Others have provided good answers but here is my answer as well:
Basically, Canon is using an outdated manufacturing technology and an outdated sensor design.
Sony achieves high DR (dynamic range) by using on-sensor ADCs (analog-to-digital converters).
Canon is still using external ADCs - and the primary reason for that appears to be that Canon is still
using an outdated CMOS manufacturing process vs the competition.
Other than that, Canon holds multiple patents for on-sensor ADCs, so they could in theory manufacture
high DR sensors.
I'm personally hoping that the 5Ds sensor is a new generation sensor and will have good DR.
It would be very disappointing otherwise.