Why did Wu Zetian want Xue Qinglin's secret letter?

Xue Qinglin's secret letter contains the plot of Wu Zetian.

1. In the TV series Detective Di Renjie, there is a plot of Wu Zetian in Xue Qinglin's secret letter, and Wu Zetian sent Di Renjie to deal with Xue Qinglin and the secret letter to prevent the secret from leaking.

2. Detective De Renjie is a China suspense TV series directed by Qian, written by Qian and starring Guanhua Liang, Lv Zhong, ZhangZijian and Xu Qian.