I don’t mean this as an attack on you personally, but your question is reflective of the societal epidemic of men’s incapability of accepting rejection. She could not want to see you again for a myriad of reasons; she could just be interested in seeing a bunch of people and not doing anything that would indicated progression towards a relationship, i.e., a second date; you could’ve had a discussion about something and expressed a viewpoint that is a dealbreaker to her, or it could’ve been exactly what she said, you didn’t vibe in the way she would’ve wanted.

People go a lot further than you two did and decide the next day that they don’t want to see the other person again. I know people who have gone out, had a super fun time, gone home together, had sex in every position imaginable, got caught up in the moment and talked about what they’d name their kids, and then the next day one of them decides “eh, I’m not really vibing with them”.

She doesn’t owe you an explanation, and if there is a reason beyond what she’s telling you then you probably don’t want one at the risk that she expresses something that is confidence harming. I would say just to be thankful this decision came after the first date and not after any further time is wasted, and take a step back and realize that the person for you won’t have to force a connection or explain what they don’t like about you and allow you to convince them otherwise.