Probably because you need to do more than just the bare minimum of decent behavior to attract her attention. If you're not showing a more than platonic interest, she's just going to assume you're not attracted to her and not give you a second thought. You need to directly express your attraction, not sit around and hope she notices you. Smile at her in a way that shows her that she is the reason you are smiling, tell her she's attractive/pretty/cute, and ask her out. It's really that simple. And no, it won't work every time because sometimes she just won't be attracted to you at all. But it [b]will[/b] work for any girl who might be able to find you appealing. You get your foot in the door by exposing yourself as a potential mate, not by becoming her friend and hoping that one day she magically becomes attracted to you.