I imagine the Kosok as being competition for the Kedashi in the skies above the Naralon Forests.
With the 'Wild' Kosok preying on the Kedashi, I just don't think the Kedashi would want to tame any of them for their own use.
The Kedashi would probably just find another (possibly smaller) bird of prey to use against the Kosoks.
(This is of course, a blatant hint for Mike to scuplt a new avian.
)
Also, whilst I would have thought it possible for any of the races to train a young Kosok, I like the idea that the Devanu are the only race big enough and strong enough to keep them under control when they are fully grown.