Because he was away in the imperial mausoleum, he didn't have any backing nor an army
He is not really that smart nor cunning enough like Zilu or his father. His motive to gain the throne was because he believe this will get him Awu back. He clearly did not think this one through since he has no acumen to lead nor forethought to see how his alliance with an enemy of the country will actually destabilize his nation. Just like Zilong or the Empress Dowager, they only think of themselves, not of the nation as a whole
Wanru's hate for Awu was an unnecessary and poorly developed deviation from the novel.
In the novel she was already married to the CP and was not raped, right? I decided not to read the novel while watching the drama so I would separate the two