They don't know the difference. Hillary and Obama are actually center right economically, but Republicans talk about them like they're commies. Sanders economic plan is considered rather centrist in Europe, and he was looked at as some off the spectrum radical. The people on the right have been lied to so long, they don't know what is left and right anymore. The left, unfortunately, is trying to catch up.
The basic theory is after Reagan decimated the Democratic party, they moved right with Bill Clinton's "New Democrats" or neoliberals, a migration right that has never stopped. Now the center, appears to be the left, to the Republicans, who have also shifted right.