To be fair, 6D was announced, when 5D3 was already on the shelves and selling like hotcakes...
Interesting. There are to my knowledge are no Canon sales figures or statements to suggest that 5D3 sales were healthy. But since you seem to have such info both I and I am sure many others would be very interested if you can share on what you base the claim that the 5D3 was "selling like hotcakes".
The most commonly quoted source are the Amazon best selling DSLR rankings. For most of the past two years, the 5DIII and the 6D have consistently been the highest selling full frame DSLRs on the list. (Since the list is constantly updated, you can't really judge by picking a single data point, but instead, you need to follow it over time to judge the trends.) In fact, for much of the past few years, the 5DIII and the 6D were the only
full frame cameras to crack the top 10.
With Nikon's recent releases, they have been doing quite well, but it does appear that they are dropping in position, most likely because pent-up demand is starting to be met. Only time will tell if the Nikons can hold their own against the Canons or if they will drop back to where they have been for most of the past two years -- which is quite far back on the listing.
As of a few minutes ago, the 6D was back to being the top selling full frame DSLR on the list, followed closely by the D750 (Numbers Seven and Eight, respectively). That's a change from when the D750 was first announced – it even held the number one spot briefly -- pent up demand. The 5DIII is, as of this writing, at #21. That's still quite impressive for a camera that has been out for quite some time, especially since it is more than five times the price of some of the cameras higher on the list.
Predictably, someone will say the Amazon list is not valid. But they will not be able to produce any comparable ranking. Amazon is the world's largest retailer and I know of no study that would show their customers and those customers' buying habits differ significantly from the general buying public.