Because he ex-husband was a felon. Once he heard that he should have looked into it, especially once the field was narrowed down and it looked like she would make the final cut.
This is just not the state's fault. Asking for his name would seem like prying and I'd also be curious as to what Juan is allowed to ask of her. I've never heard a prosecutor ask for the name of the person a juror knows who's been charged/convicted of a crime. Never.