It's all a consequence of the First Past the Post system. In the US, a Presidential candidate wins if they get a majority of the electoral votes. If you have 3 candidates in the election, the 2 candidates who are more similar to each other draw the most from each other's base; thus, the 3rd is at an advantage. The 2 candidates who are more similar are therefore incentivized to consolidate into 1 candidate through compromise. 2 candidates -> 2 campaigns -> 2 political parties leading those campaigns.
In order to run a political campaign, you need lots of money and lots of people. Each party has to be amenable to both capital interests and an ideologically motivated base of people. Right wing politics are inherently amenable to the capital class, but the left wing politics are not. Hence, you get a rabidly cultish Republican Party and a very weak, anemic Democratic party built on a web of tenuous compromises between the rich and the poor.
