Because when it comes to Sasuke; Naruto doesn't defend his woman. And then if he "does" he says something insignificant and moves on like nothing happened.
Sasuke can do anything to Sakura right in front of Naruto's eyes, and Naruto will go "Oh it's ok Sasuke. You're still my best friend."
Man, you people are not thinking from Naruto's pov. You are looking at it from Sakura's pov.
Just think about this from Naruto's pov-
You have a guy friend and a girl friend. You really like the girl but you know she likes your friend. Your guy friend does not like her. You see that your girl friend is sad about this and suffers because of this. You see her thinking about and crying over the guy many times. You reach a conclusion that she likes him a lot. You just want to see her happy and you decide to back-off and just become a supporting friend who is there for her. You also know that she knows you like her. She has even made some sort of confession about loving you. You are in total confusion. You are not sure where you stand in this issue.
I ask you- Would you butt in and interfere in this? You know she likes him a lot. Can you really be rude to the other guy and defend her? Wouldn't you be worried that she might not like you doing that? Its not an easy situation.
And after all this, you see her again saying - " If there is a space in your heart for me......." to that other guy.....................
There is nothing you can do except silently watch on and let those two resolve this issue themselves.