Why does she never initiate the conversation?

So theres this girl i've been talking to for a while now. On facebook and text cuz i never see her at school. So whenever we talk so like 6 times weve talked straight for 3 hours plus. And she allways replies so fast! Which a lot of people say she doesnt do. Shes not the most popular girl at school, she stays really low key not like me im really popular. So we never get to see eachother. So anyways whenever we talk the conversation just magically keeps flowing somehow. One thing is one time she said i gotta go after we were talking for three hours so i said time flies doesnt it and she said :). So i said text me later and she said i dont text unless i have to, and i said well u have to. She said no i dont. I said yes you do. And she was like i got better things to do :P. I said no you dont and i just left it at that. So the thing is i tried not talking to her for a few days and what ive realised is she never starts the conversation. Why?