Nominating a more conservative candidate because she or he is the more electable Democrat is sometimes necessary, but nominating more conservative Democrats who can't win anyway is pointless. This is not just an abstract point, because candidates like Blanche Lincoln, even if she were to win, often stand in the way of successfully passing legislation once they are elected. Therefore, if they are not more electable in November, there is very little strategic reason to nominate these types of candidates in primaries.
In an election like Lincoln's the general disdain for primary challengers which is held almost universally by political elites is also probably a factor which pushes people like Bill Clinton to support her. This demonstrates the enduring strength and allure of the insider political culture and the deep fear of a primary challenge which many elected officials fear. After all, if people like Lincoln lose primaries simply because they lose touch with voters, than almost anybody would be vulnerable to a primary challenge. The word for that is democracy; and it is disturbing to again see how many politicians are afraid of it.