Because the Vietnam era sank 4 of best Democrats out there: MLK, Malcolm X, RFK and JFK. But the only 2 that the conservative movement really cared about were RFK and MLK who would have pernamently set the Democrats in power.
And the conservative movement wanted the Reagan Revolution. But the Liberal Era will be back in 2018.
Malcom X was not a Democrat - far from it as we can see in his "Ballot or the Bullet" speech. Nor was King, who moved from offically nonpartisan into vauge socialism at end of his life.
'Nam was largely JFK's fault.
RFK was trailing Nixon even worse than McCarthy in 1968.