It’s intentional. As Noam Chomsky said:

The smart way to keep people passive and obedient is to strictly limit the spectrum of acceptable opinion, but allow very lively debate within that spectrum

There has been a deliberate effort to limit the spectrum of acceptable opinion to relatively moderate liberalism (the left) and relatively moderate conservatism (the right). The intent is to create the illusion that no political ideologies or possibilities exist outside of this narrow spectrum.

Breaking News: Americans are idiots