AOK units are animated 2d renderings. AOM units on the other hand are actual 3d models.
In AOK, when a unit died, it went through a few phases, firstly, the animation of the death, when it hits the ground. Secondly, an ES artist had to paint blood onto the bitmap, and then thirdly, I don't know whether or not AOK death animations were done with some kind of specializied generator by hand, but the animations for 'decay' were only 5 images long, and were simply the last frame of the death animation edited into a skeletal figure.
In SWGB, the artists simply made the decay frames for the units a very slow and tacky animation of them sinking into the ground, which works, because most star wars games like to avoid blood, but it also meant they didn't have to do any work at all when it came to making them decay.
In AOM, they didnt have that luxury, if they wanted to make the units 'rot' into a skeleton, they'd have to make new models with blood edited on, and make even more models of the skeletal versions, which I think would take forever, especially since no one will even care about the decay frames, though they are nice to look at, since they give you glimpse of the battlefield. This is because of the fact that the units are 3D models. So what they did was animate their death then add a seperate little animation for blood, which seems to be shared among most units, (the scarab uses its own blood sequence), and then make a "tracing" of the last death frame, when units would die the tracing would appear under them, and the AOM units would thus sink into the image, giving an effect that they decayed, or something.
In AOE3... it will be even harder to do "decay" frames because of the new physics engine they will incorporate. In AOM the units died in a set way, so they could just simply trace the outline of the body, but in AOE3, they dont have that luxury because every unit will die in a different way. So I dont blame them for making units 'fade' instead of decay, because it would take an awful lot of work to make a dead body for every single unit.
In WC3, they had a (seemingly) simpler system which I thought looked pretty good, but I dont feel like explaining it.