With the pawn on the seventh and the defending king far away, the attacker brings the knight to attack the blockading knight or to control the squares it must retreat to. The lone knight cannot hold the promotion square forever.
The black knight on d8 blocks the pawn, but Black's king on a1 is far away, so the knight has to do everything by itself. White's winning method is to bring the second knight into the fight: Nf3 heading for e5 and then f7 or c6, squares from which it attacks d8 or covers the knight's escape routes.