Because you're going to lose your rook if you take white's rook. After Nxa1 white plays Nc7+ and you're only up a pawn. With Nxd4+ you pick up another pawn and white has to capture with the b5 knight.
Because you're going to lose your rook if you take white's rook. After Nxa1 white plays Nc7+ and you're only up a pawn. With Nxd4+ you pick up another pawn and white has to capture with the b5 knight.