Logic and mathematics is why you only get two choices. Anybody at all can start their own party. Then you have to gain enough support to get on the national stage. Then you have to get enough votes to have a majority over all the other parties. In the end, it will come down to two candidates. If you think about it, if you have three strong parties, and one got the most votes, then theoretically 2/3 of the voting public did not get the candidate they wanted and would be alienated. Then you no longer have "majority rules" which is the cornerstone of Democracy. Then you only have "most votes rule". If you have five or ten viable parties, such as Italy, then you stand the chance of a government voted in by maybe 1/5th of the voting public, possibly 1/10th of the population. The only logical way to keep from chaos is to work towards viable change within one of the parties. Bernie Sanders may have a chance to do that. Garry Johnson is a sweetheart of a guy and his chance of building a viable party is zilch.
That being said, the way Trump has splintered the Republican Party, it is not out of the realm of possibility that they might split into two parties : The Old Republican Party and The Alternative Hate Party, but, of course, I say that in jest.