There’s a big misconception right here. You seem to accept the right-wing dogma that America’s present division is between a “right” and a “left”. Instead (to the extent that right- and left- labels are meaningful) you have a far-right Republican norm, and a centrist (or even right-of-center) Democrat norm. “Centrist” is more or less a synonym of “moderate.” You seem to be asking why these moderates are moderate; the answer is … it’s because they’re … moderate.
True left-wingers are easy to recognize in the American debate: they tend to regard Republicans and Democrats as equivalent.
This all should be very obvious, yet even the otherwise-intelligent SDMB insists on following the FoxNews/Limbaugh lead and labeling moderates as leftists. When this is pointed out, Dopers reply, “left-wing by American standards!” but calling a goat a pig doesn’t make it a pig. Calling a moderate a leftist doesn’t make him one.
Hope this helps, though am doubtful. 