I don’t know what country you are from or how your voting system works. But I will guess that your country has many parties and after the election, a governing coalition is formed.

In the US voting system, similar parties get punished by stealing votes from each other. So, in effect, we have to form our coalitions before the election and choose the single candidate that will stand for all of us. So, you can think of the Democratic Party as the Democratic Coalition, made up of some truly left-wing factions, as well as some not very left-wing or even centrist factions, and so our candidate will be much more watered down than what you’d see in a different system.