I'm not sure what you are implying about what broader significance 1996 or 1998 had in terms of dramatic declines in these groups.
For the same reason that if this graph continued, the number would drop sharply after the 2012 election. It was widely reported in early-2014 that the number of right-wing militias and hate groups dropped once Obama won.
For the same reason, Republican rhetoric is not nearly as bad as it was 2009-2012 (though still pretty bad): because Obama owned them. They are defeated, so they have resigned themselves to waiting out their Presidencies until the next Republican comes along.
Clinton and Obama were/are seen by conservatives as anti-American (more Obama on this point), culturally liberal, and pro-gun control, so militia groups increased during their first terms and sharply declined once they won re-election.
Am I saying that all (or even a majority) of Republicans and conservatives are foaming-at-the-mouth militia members? No, not at all. But the reactionary wing in this country is very vocal and
very, very paranoid, and easily whipped up into paranoid fear by conservative media and politicians when a liberal Democrat associated with gun control and other liberal issues gets into office.
It's really that simple.
