The reason it quite simple - so many things that are available to BM hunter are related to KC, however since it does nearly the same as the filler - it invalidates all those choices.
Talents
Killer instinct: increases dmg of KC by 50% when under 35%. Cool, except since it hits like a wet noodle you are better off going with one of the other talents because they give you better benefits throughout the whole fight.
Killer Cobra: cobra shot finishes off the cd of KC. Cool, let’s us use a move that hits for a bit more than the move that resets it’s cd a few more times during bestial wrath. Both other choices far outweigh this since it hits for nearly the same as the other regular abilities.
Mechanics
Cobra shot reduces cd of KC by 1 second. Cool, however for me at lvl 54 cobra shot hits for 798 and KC hits for 935… and that’s with 39% mastery - meaning it’s base dmg would be 570, which is much less than cobra shot baseline. This makes no sense.
Legendaries
Qa’Pla - barbed shot reduces cd of KC by 5 secs. Awesome! That is… if KC did some real dmg. As it stands a legendary power lets you use a move a bit more often that hits for a bit more dmg than your filler.
Flamewaker’s - Cobra shot has chance to make KC take no focus. Cool, we can use our filler a bit more often because we won’t use as much focus.
Conclusion
Essentially a weak KC effects so much about the class. It’s base dmg with NO mastery should at the bare minimum deal 10% more than the filler cobra shot. This is not to say cobra shot needs to be nerfed, but KC needs to buffed by quite a bit.