The Populist Party pushed for reforms that many people favored. What do you think causes such third parties to form, and
why do they tend not to last long in American politics?

Third parties usually form when a sizeable group of people begin to organize around an issue that is not being addressed in a way they like (or at all) by either of the two major parties. If the idea(s) of the new party become popular enough, the major parties will take notice and begin to support these ideas. Once a more established and stronger political party takes on the cause of the third party, the reason it was established and the need for it disappear.