Maybe she just wanted to be independent. Married women became like minors and their husbands managed their assets. Or maybe, with a face like that and a father who was still alive and well and could leave his money to Mrs. Borden, she didn't have any serious suitors, or not of the right kind.

A family like that was likely to have standards. Maybe they wanted a potential husband to be rich, educated, from the area or willing to stay, plus who knows what else they wanted, and a man exactly like that and who would marry Lizzie just wasn't easy to find. Then she got older, and past 25 or so a woman was considered an old maid.

There could be a number of reasons like that rather than some implausible story of sexual abuse. She wasn't even attractive.