A knight that must guard a rook pawn's promotion square from the only two available squares gets attacked by the king and has nowhere to go. Rook pawns on the seventh usually beat a lone knight.
The pawn on a2 promotes on a1. The squares from which a knight can control a1 are b3 and c2, and both are adjacent to the black king on b2. White's knight on b3 is already attacked and has no safe square that keeps a1 covered: c1 is attacked by the king, c2 is attacked by the king, and d4, d2, a5, a1 (!), c5 all give up a1. Whatever White does, Black promotes next move or wins the knight.