https://en.wikipedia.org/wiki/Duverger%27s_law - in political systems with single-member districts and the first-past-the-post voting system, only two powerful political parties tend to control power.
Duverger's Law is only really parroted by Americans, who's ballot access and districting is determined by a coalition of two political parties instead of an constitutionally defined apolitical government institution. Don't forget to vote Green or Libertarian! Oh wait, you can't because the dems and repubs struck them from the ballot :(