1- They're afraid of what will happen if they leave.
2- They might not know what a normal relationship is, and think abuse is normal.
3- If they're in an LGBTQ relationship and hasn't come out yet, the other person might threaten to out them or 'expose' them.
4- It can be embarrassing to admit you're being abused or shameful.
5- If you're in an abusive relationship, your self esteem is constantly being brought down, when you get out of the relationship you won't have much confidence.
6- They often feel for their partner, even if they have abused them.
7- Lack of money.
Hope I could help!