Frankly, because with any grasp of republics of the past and how they died, it's hard not to read this as treasonous. Quite possibly treasonous to the point that someone actually stands up and does something about it, as opposed to, you know, the relatively "minor" acts of sedition he's been guilty of up to this point.
Refusal to accept the democratic process is contrary to the idea of democracy itself. If his followers act on that, and he is judged as having colluded with them (as is becoming increasingly likely) or providing support, he exposes himself to a federal death sentence.
Sadly, even if he was put to death for treason, the scars on our republic would be unavoidable. As soon as he opts out of accepting the results of a democratic election and isn't immediately denounced and shunned entirely by society as a whole, the damage has been done.