If she had left him, she knew he would get unsupervised visits with the kids, Knowing him the way she did, that was probably enough to keep her there to protect them.
In the end, her fears were justified. She didn't even leave him yet, he "destroyed" her and her sons.
As far as why she didn't tell anyone, IIRC, she did voice some of her fears to her friends and family. Her family urged her to have a secret bank account, also IIRC, as a result.
She knew this monster. She had a gut feeling as to what he was capable of, based on knowing him, and based on her feelings. But telling a court that he said, "If you leave me, I'll destroy and the kids will have no parents" would have been unlikely to restrict his visitation rights to his kids much. He seemed good at hiding his real self from most people. Susan would have had a very difficult time protecting herself or her kids had she tried to leave earlier, based on her feelings and an allegation as to what he stated would happen if she tried to leave. The court would have demanded much more.