A knight separated from its king is hunted down: the rook cuts off its return squares and the attacking king helps trap it, or the defending king is mated while the knight watches.
The black knight on a2 is on the opposite side of the board from its king, and that is fatal. A knight far from its king has nothing to protect it, and a rook plus king can trap it on the edge or win it with a double attack while mating threats against the king tie Black down.