Because women think based more on emotion than logic. As time goes by, a relationship strengthens even if one of the person's involved doesn't not want a formal attachment. Maybe she had hope that things would change as the relationship progressed. There is nothing wrong with having a little hope for some kind of future.
It is commendable that you are up front with your feelings, and you should definately stand your ground. IMO if two people truly love each other, a relationship can be wonderful even without the formality of marriage. In some cases, it just puts a damper on things when one or both people start taking each other for granted because there is some piece of paper that states they are married.
If you promise your heart to her and you are faithful, I feel that should be enough, However, if you are in a relationship, but you still always put yourself first and act like your single (for the most part), then you really aren't in a true "relationship" anyway.