Against a rook pawn the knight has fewer squares to work with. It must reach the blockading squares with check and keep returning; a single inaccurate move and the pawn promotes.
The rook pawn on a4 is a special headache for a knight because half of the squares around the a1 corner are off the board. The knight on e1 has exactly one way to hold: Nd3+, attacking the king on b2 with check and heading for the squares b2, c1, c3 that control a1 or a2.