Problem is, if she did, she would be the prime suspect. The reason she can allow herself to do it now is because: 1. she is desperate, and 2. she has means to place the blame on someone else due to recent events and circumstances. Had she done this at the start of the story the imperial blade will clearly be pointed at her neck. In her eyes, she still has the upper hand, but the most to lose.
She is too desperate for her own good. The way I see it she craves power and wants her son to be the crown prince. But ever since the incident that happened with her son, she became power hungry
if it is this easy to claim the throne, the empress should have killed the emperor earlier and save us 200 chapters...
jk