There are two basic reasons why a Red State Democratic Senator might feel the need to maintain a moderate or conservative public profile (it's always possible that this sort of Senator often is less liberal, but lets at least assume they would like to be more liberal than they think they can get away with and still win reelection). The first is that such a Senator needs to win the votes of a more conservative electorate. The second is that such a senator has a much smaller in-state donor class of ideological liberals. To mitigate the first requires that you be a "different kind of Democrat"; pro-life, pro-gun, pro-military-industrial-complex, whatever. To mitigate the second, though, requires that you kowtow to whatever interest group is willing to fill your campaign coffers. And interest groups know what's up; they're not going to be fooled by a vote against final passage if it comes right after a vote for cloture.
When you see Ben Nelson or Evan Bayh or Mary Landrieu standing up for the poor, downtrodden, put-upon health insurance industry, and wonder why Harry Reid doesn't just tell them to vote for cloture but against passage to let them show off their "independence", ask yourself whether the Centrists are trying to appear Centrist in the eyes of their constituents or the eyes of their donors.