Because she did not feel like yes was the right thing for her.
It is her prerogative and you must respect that. If you thought the answer was going to be yes, you missed something. If its important to you to find out what made her take that option, talk to her openly and rationally and get satisfactory closure. If its not important to you to take the time to find out then you have answered your own question.
Life goes on mate, get over it you must. I have and I am only11
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She might be scared to move onto another step, so just give her some time to settle.
Maybe you aren't asking the right question. Make it about what she wants to do, not what you want. Maybe she isn't ready. Talk about it and be ready to accept no as an answer.
That is besides the point, if a girl says no it can only mean no. wether you like it or not.