Because, unlike people who are bought into the whole red-blue sandwich strategy to dominate our people, most South Dakotans are normal, and the only thing normal people hate worse than Democrats is Republicans.
Up close, there is probably no justification for voting for John Thune, but he does have access to goodies (as most Senators do), so why not re-elect him?
Once he has to be replaced, though, the next empty-headed pretty boy (or girl) brings nothing to the table except Republican policy ideas, which most people despise, so another 51-49 election goes 49-51, and the Great Game goes on while the people suffer.
“...except Republican policy ideas, which most people despise.”
Limited government, freedom, family, keeping what you earn, defence of innocent life. Tell me “normal” person, which of those do you despise?