It's perfectly understandable and makes all the sense in the world. Americans would never elect a socialist. At least not in the near future. Now they will elect Republicans and Democrats. If you're a socialist, running as a socialist, only 45% of all Americans would consider voting for you. That means 55% wouldn't under any circumstances. Of that 45% who would consider voting for a socialist, the key word is consider, that doesn't mean they would. https://news.gallup.com/poll/285563/socialism-atheism-political-liabilities.aspx
Then under our two party system, it is designed not to give any other party outside of republicans and democrats a chance of winning. Republicans and Democrats write our election laws and they do so as a mutual protection act. If there's one thing each major party agrees on, it's no viable third party will ever rise to challenge them.
A socialist running as a democrat is a no brainier. As far as I can tell, see, find is that the socialist party made it onto just one state ballot for 2020, New Mexico. You're not going to win the presidency being on only one state ballot. Democrats are on all 50 states and D.C. as are Republicans. Democrats and Republicans are automatically ballot qualified in all 50 states. Every other third party has to jump through a million and one hoops just to get on the ballot. https://ballotpedia.org/Socialist_Party_USA