because he was a domineering brute! from The Guardian:
How could Josef Fritzl's wife not have known?
Police quickly ruled out the possibility that Rosemarie Fritzl had any knowledge of her husband's activities after details of his horrific crimes emerged.
The implication is that she never had any reason to go to the cellar or, if she did, she was discouraged by her husband without him raising any suspicions.
Rosemarie's sister, Christine, has come forward to reveal Fritzl would often spend the whole evening in the cellar, but she corroborated the claim that Rosemarie knew nothing of what went on underneath her own home. "Often, he would spend whole nights down there," Christine told an Austrian newspaper. "Rosi wasn't even allowed to bring him a coffee." She described her older sister as "dominated and constantly belittled in public" by Fritzl.
However, it has been difficult for police to accept Rosemarie was completely ignorant of her husband's double life. "It defies logical thinking that a woman who has had seven children with her husband would make it possible for him to have anther relationship with his daughter and have another seven children," Franz Pölzer, the head of the criminal investigations unit in the province of Lower Austria, said.
However, he added: "We still find it hard to believe that no one no neighbour, family member or acquaintance noticed anything."
i think she was scared to death of him & pretty much just kept her head down & accepted everything he told her...i really don't think she was a strong person. i suspect he bullied her relentlessly & she was simply afraid of him, afraid to question him or anything he was up to.