英语 强调句的否定转移It was because her mother was ill that she didn`t

英语 强调句的否定转移
It was because her mother was ill that she didn`t go to work
强调句型 若从句是否定形式 那么 应该否定转移至主句，但此句为何没转移？是不是应该because的原因？
书上有个例子
8. 使用强调句型时 须注意否定转移 如
We can`t expect to succeed without an effort
It is not without an effort that we can expect to succeed.
所以 强调句的否定转移是有的
实际上有强调句的否定转移一说么 ？