Frankly, because he's been our worst defenseman, covered up by his physical play and fighting attracting positive fan and media attention, and some spectacular on-ice luck, the sort of which is carrying us to win games that we really shouldn't.
Of all the players, he's the one who most desperately needs to be sent down to the AHL, which is not a slight against him in the least, but a necessity for such a young and raw player.
Kovacevic, on the other hand, has developed into a player ready for the NHL - one who is generally carries the load of most pairings on which he has found himself and is among the best defensemen that we have at suppressing shots and scoring chances.