Re: WHY does NOT count? knocked down enemy but killed by OTHER enemy .

That's how it works right now. If you knock someone down you will get the kill.
The only time this doesn't happen is if that player gets revived from knocked down state.

It doesn't matter who finishes the kill, the player who knocked them down gets the credit.
You're asking for something that's already in game (unless I'm mis-interpreting your poor English)

EDIT:

It's possible you may be asking to be able to kill-steal from another team after they knock down? (a-la PUBG). If that's the case, then I'd disagree with that too.