Why does the united states have only two major parties?

the constitution prohibits minor parties

the first-past-the-post electoral system

state laws forbid minor parties

two parties represent all the choices voters need

The Constitution doesn't prohibit other parties, and nor do States, as there do exist other parties. First-past-the-post is just a winner-takes-all scenario, and doesn't really have to do with political parties. So lastly, most of the choices voters believe in typically line up with one of the two opposing parties. So, I believe the answer is D.

i confess that there are several parts of this constitution which i do not at present approve, but i am not sure i shall never approve them: for having lived long, i have experienced many instances of being obliged by better information or fuller consideration, to change opinions even on important subjects, which i once thought right, but found to be otherwise. it is therefore that the older i grow, the more apt i am to doubt my own judgment, and to pay more respect to the judgment of others.