Because true authoritarians are always insecure of their power, are ruthless in their methods to retain it and would simply not allow their enemies the time to plan and organize their defenses or attack that would threaten it... if anything Brennan has acted more characteristic of an authoritarian because he never even gave Trump due process before accusing him of being a Russian agent, treated him as guilty before proven innocent and advocated to remove him from office immediately.
And here we are more than a year later, with still no concrete evidence to this grand claim. While the accusations against Brennan, being in charge of Spygate, have only came out recently. We'll have to wait and see if they are able to produce the evidence for that.