South Dakota isn't deep red. Just ask Never Trumper John Thune.
I believe the Senate should always have been Majority conservative. A majority of states has always been conservative. Why didn't the makeup of the Senate reflect that fact?
The best explanation I can come up with is there are few enough Senators elected each cycle (33-34) that the Progressive Media and allies can focus enough resources to get and keep RINOs or other Progressives in office in those several states, if the population knew there political philosophy.
John McCain is a clear example.
He repeatedly lied, again and again, ran as a Conservative, then ruled as Progressive, for decades. Fully supported by the Progressive media in Arizona and the U.S.
They also had Tim Johnson a couple of cycles.