Hey been there done that a lot, several reasons I've observed.
The main theme is that she is consciously or subconsciously comparing you with guys she's had sex with and how fast they got there (ie, if the last few guys she dated got her in bed fast, a slow escalation may stick out in comparison). You typically will see virgins hang around afterwards more frequently, case in point.
Chase writes articles about how girls tend to view guys who TRY and AREN'T able to sleep with them as having "failed" the mating call in her eyes. That's not to say you've failed, but rather her view biologically. I have had numerous cases where I tried to get with girls, but failed, and 80% of these cases never saw them again. The ones that I do see stay around after are typically the ones looking for a husband/extremely serious relationship, or ones where my value is perceived to be much higher ("ie, asian girls in Asia due to wealth disparity").
HOWEVER, I have also had cases in the past where I turned the girls down for sex (crazy now thinking about it I know but due to things like not having protection, not sure if she had STDS, too drunk, etc from my younger days). In almost all these cases, the girls would almost always agree to another date. The reasoning here I believe was that they saw it as ME who was the person doing the rejecting, not them.
If I were you and you somehow absolutely didn't want to have sex on the first date, I would definitely frame it explicitly as YOU being the one not wanting sex. To date, the BEST excuse I've used is "If I sleep with you on the first date, I will probably lose interest in you right away", best done after escalation and before she puts up a wall. This frames you as a guy in control, and near zero guys will ever use this line, so will make you stand out. I've used this line around 10 times now in the past and each time the girl agreed enthusiastically to another date.
These are just my experiences and impressions.