Since partner doesn't hold the J or 10, covering cannot promote a trick in his hand.
Another way to look at it: if the 9 is singleton, then declarer holds AK6 and covering the 10 will set up a fourth trick for him with the J and 8, while ducking holds him to 3 tricks (A, K, 10). If the 9 is doubleton from 96, then covering is a disaster since declarer holds stiff AK.