I refer you to this tweet thread here, which I quote below in an image:
To put it simply: the structure and incentives implicit in democracy produces an organization that, while it may occasionally do something useful, spends most of its time flailing back and forth like a drunken idiot, unable to make up the screaming chaos of its mind.
The eldrae, who believe very strongly in excellence, competence, and harmony, wouldn’t tolerate it for a moment.
Hence the structure of the Empire, which hinges on putting the absolute best and most well-trained person it can, after strict and stringent testing, in the Big Chair, and letting them run things in accordance with the Charter. They are well provided with technocrats (the Imperial Service) and advisors (the Senate and the Privy Council) but are otherwise empowered with all authority necessary to do the damn job.
There are checks and balances in the form of the Curia, the ability to remove diarchs who prove incompetent or go crazy on the job, and ultimately the ability to call a vote to dissolve the entire Empire – but, you see, the theory is that you can have such checks and balances without having to infect your entire governance with institutional schizophrenia.