#1 She’s on her period. You saw it coming, but you didn’t expect it to be that blunt. Most women will refuse s_x when they are on their period. Some may concede and allow it, but that usually applies to women who are in relationships or who just plainly enjoy period s_x.
#2 She doesn’t like you.
It’s the most plausible reason out there. You can’t force someone to sleep with you if they aren’t attracted to you. The date may have gone well, but you never know what a woman is thinking until you realize that you won’t be getting a second date.