For certain reasons, your girlfriend may not be ready to commit into a marriage as well but that does not mean that she will think like this in the future as well because her thinking may change with the time too.
Getting married too early
How long have you been together with her? The rush of getting into marriage maybe too soon for a relationship that is not too long together. She may need more time to understand you better before making this decision.
Not ready for commitments
Are both of you financially stable to set your own family? There are many commitments in a marriage such as house, children, household expenses, etc. If she is not ready for that, she may not want to get married.
Fear for marriage
She is still thinking of whether you are the right man for her. Maybe both of you have always been quarrelling or she finds that the relationship is plain and boring.
Assurance is always needed in a relationship and if you are sure that she is the only woman that you want to marry, you should make her feel confident about this relationship. Give yourself this chance to discover the proven ways on how to better a relationship and change her opinion in marriage.