Because if you pin the knight before the opponent castled he has the chance to push his king side pawns forward and almost trap your bishop. Something like this.
In the ruy lopez black plays a6 so that he can later remove the pin with b5 as well as gain space on the queenside.
This move is always debated...is moving this pawn safe, considering it weakens the pawn structure. Most of us players played this move to scare the opponents bishop away if it pins our knights on c3,f4,c6,and f6 respectively. Emanuel Lasker's states that we should not pin our opponent's knight against its queen before the opponent has castled. Why did he state this? Pls Help....