First of all, I would note that there are various ideas about why the Socialist Party failed in the US. There is no way to prove which one is correct. You should check in your textbook or notes to see what answer your particular teacher expects you to give.
One view is that socialism failed because socialism goes against the grain of American thinking. From this point of view, Americans are a people who love the idea of being able to get rich. When they are poor, they do not want to pull the rich down. Instead, they want to have a chance to end up being like the rich people. Since socialism is more about pulling down the rich and ensuring that no one becomes truly wealthy, socialism failed.
Another point of view is that socialism failed because the capitalist class fooled the working class into being divided among themselves. In this point of view, workers should naturally unite with one another and, being more numerous than the rich, they should be able to take over the government. The problem is that the capitalists can fool them into refusing to unite. The capitalists can do this by, for example, playing workers of different ethnic groups against one another. From this point of view, socialism failed because the workers were divided between white and black, between immigrant and native-born, and between types of white immigrants.
Finally, there is the argument that socialism failed because our capitalist system moved towards socialism enough to mollify angry workers. In other words, when workers were being abused, or when the country became really poor during the Great Depression, the government passed laws to help the workers. The government passed things like minimum wage and maximum hours laws and it created programs like Social Security. This kept workers from being angry enough to become socialist.
Any or all of these might have had something to do with why the Socialist Party failed. There is no way to know for sure which of these is truly correct. Again, please check your text and notes to see if you can determine which answer your teacher expects.