A knight can attack pawns on either colour; a bishop cannot. When the defender's pawn is fixed on the colour of their own bishop, the knight hits it and the bishop can only babysit.
Here the pieces change roles. Black's pawn on d5 is a light square and so is Black's bishop; the bishop can defend the pawn but can never attack White's d4 pawn (dark) or do anything active. White's knight, by contrast, can hop to e3 and attack d5, and the black bishop must answer from behind.