Because they focus too much on the means and the desired ends. The how and not the why.
This. Hitler used "big government" to achieve his goals, and "big government = left wing" in the minds of a lot of American conservatives.
I guess the traditional libertarian/traditionalist ideology would be a good contrast to fascism or at least it would in the right wing. Basically "Fascism" can be a somewhat useful way of describing everything that is "well right of the center" but not "conservative". Its almost as if at least Hitler's fascism was trying to achieve through state action what most right of center policies naturally and inevitably achieve. That is, capital controlling the state instead of state controlling capital. What conservatives do is try to enforce a separation of capital and state that inevitably leads to the control of the state by capital.
Therefore I think-
Left of Center- More control of capital by the state
Conservative Right of Center, Classical Liberalism, Libertarianism- Separation of Capital by the State. Modern Liberals, modern Conservatives, and traditional conservatives debate whether its more important to dejure maintain the separation or the defacto one even if it means relaxing the the dejure separation.
Those that are right-of-center but not "conservative"-More control of the state by capital
Conservatives are partially right when they way they accuse of Fascists of being "liberals" or "socialists" in that they are "not conservative" and don't want a separation of capital and state.
Liberals and left of center people are right when they accuse Fascists of being "right-wing" because they they believe in the state being controlled by capital.
