[xg15]

Not a US citizen, but my understanding is that the two-party system is not mandated by the constitution, or even encouraged. It's something that developed on top of it, as a consequence of some bad rules (first-past-the-post as you say) but also other factors (campaign financing)

Because the beneficiaries of the system get to make the rules, there is also a push to change regulations more and more to favor the two-party system.

There are other parties (e.g. greens) it just doesn't make any practical sense to vote for them.

[kristofferR]

Your first and your last sentence contradicts each other, in my view. The reason why it doesn't make sense to vote for the greens is the system laid out in the constitution.

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