Notice how the number of Republicans and Democrats who had similar voting records has decreased and how red and blue have pulled apart.
Republicans, as well as Democrats, tend more and more to vote the same way as the rest of their party, in spite of all the talk about Flake or McCain or Corker.
Right now the Democrats are more organized, drilled, and obedient.
That's because they are out of office and only have to vote "no."
Republicans don't have anything to offer them that could outweigh what their own party leadership and the ideologically energized donors and party base can.
When Democrats were in the White House and controlled Congress and had big plans there were a few more mavericks, dissenters, and drop-outs in their party, just as there are among Republicans now.