Manipulation, being afraid, denial…
All of the above? Not necessarily Stockholm Syndrome, but often the sufferers make up reasons why they deserve the abuse - they pissed off the abuser, so it wasn’t like they didn’t deserve it.
Or you get to such a low point where you don’t think anyone will ever love you but your abuser - often that’s what the abuser has been telling you for a good while too.
Or you think you can save the abuser. It didn’t start out like this, so maybe it’ll go back to being all good again. They just need to get over this bump and then the abuse will stop.
There can also be threats. Maybe the abuser has threatened to harm the abused, hurt their family, out something that could ruin their life… You name it.
So it can be several reasons at the same time. Also, the abuser can be so good at apologising the abused believes they’ll never do it again. And when they do, the abuser will just apologise all over again.