From wikipedia

The SPD was the largest party during the first 13 years of the new Weimar Republic. It decisively won the 1919 federal election with 37.9 per cent of votes, and Ebert became the first president in February.[17] The position of chancellor was held by Social Democrats until the 1920 federal election, when the SPD lost a substantial portion of its support, falling to 22 per cent of votes. After this, the SPD yielded the chancellery to other parties, although it remained part of the government until 1924. Ebert died in 1925 and was succeeded by conservative Paul von Hindenburg.

imagine being so shit at ruling, people decide to vote for conservatives and fascists instead