Why do you think Delia stays with Sykes for 15 years?

I think Delia was so beaten down that she didn't know or deserve anything different. After women have been beaten down and ground up by the abuse, men then hate them even more for being a useless obstruction and a reminder of their failures. This is key to understanding how the relationship between Sykes and Delia may represent relationships between men and women more broadly. Instead of being grateful to Delia because she supports the two of them, Sykes instead hates and abuses her, perhaps feeling emasculated. Clarke's analogy suggests that the more a man abuses a woman, the harder it is for him to stop, because she reminds him of his failures. This certainly seems to be the case for Sykes.