She said no to a relationship why?

I’ve been on four dates with her and spent the weekend with her family. We have a lot in common and I click well with her family. We are traveling to Europe for a week and I asked if she was interested in being in a relationship. She said it is too soon for that. She said a few dates back that she has a boyfriend by the third date on average. We are the same age.
She said no to a relationship why?
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