Theres no mystery. Mitch McConnell’s GOPe scuttled him.
That is only part of it.
Remember the voting booth irregularities, and Republican Secretary of State's refusal to secure the votes for recounts when evidence of involuntary vote switching was very apparent, and to investigate the crime. If they use the kinds of voting machines that could be hacked externally it is little wonder they have the liberal (D) in there now.
Maybe Jeff Sessions can redeem himself and run for his old job again, for what ever worthless service he was to Trump these past 2 years.