> ...both parties are different. But combined, their views represent only a narrow slice of the political spectrum.

That is (by way of the Median Voter Theorem) a consequence of the two-party system, which is (by way of Duverger's law) a consequence of winner-takes-all or first-past-the-post (and not proportional) voting, I'd say.

https://en.wikipedia.org/wiki/Median_voter_theorem

https://en.wikipedia.org/wiki/Two-party_system

https://en.wikipedia.org/wiki/Duverger%27s_law

https://en.wikipedia.org/wiki/First-past-the-post_voting