The Amazon metrics do not have any real basis as the driver is really new camera releases, which Canon for the most part has been limited in the last few years at the top end of their lines which is where Canon really drives sales - i.e. 1Dx, 5D MKIII, 7D. Yes there has been 6D, 60D, 70D, t5i, etc... and say with the 70D, I think a lot of that has been for video, but again, what I see as a general formula is
New top body = sales (X) +.5x Prior Body Upgrade.
As an example, when the 810 came out I saw a ton of 800s for sale, just as there were a ton of 5D MK IIs for sale, when the 5D MK III came out.
Even though a lot of people have moved on from the 7D, a lot of people have held on or maybe gone a bridge model, but if Canon releases a decent replacement, you can expect to see not only a lot of sales of that camera, but a lot of 7Ds as well.
Amazon also tends to be more of a consumer market than a Pro / Prosumer - as a point of reference, no 1Dx of D4 D4s D3 D3x in the top 100.
A good point, Canon's path of less frequent upgrades likely means sales get compressed into smaller time frames. In the last 12 months Nikon have released the D4s, D810, D610, Df, D7100, D5300, D3300, V3, J4 and S2 where as Canon have only released the 70D, 1200D and M2.
The production delays for the 70D sensor are I'd guess a big factor here, in the next 12 months we'll likely see a 7D mk2, a Rebel and 1-2 M bodies with that sensor or something similar plus potentially FF updates as well.