The SPD used to be radical, you know. All the social-democratic parties were.

Electoralism deradicalizes and demobilizes the working class, as the means become ends themselves. The politicians end up the leaders of the movement through their position as figureheads and spokesmen for "the people's demands. To be a politician you must strive to be electable. You gotta satisfy wealthy donors to finance your campaigns. This puts pressure on you to make your platform moderate. This is what happened to most of the social-democratic parties of Europe.

Furthermore, the structure of state and the economy means that the unelected state bureaucracy (such as the various managers and officials in the various governments and agencies, or the armed forces) can struggle against you and stifle reforms, while capitalists can use the threat of disinvestment, capital flight, and capital strikes to pressure you, seeing as how they control the economy and you rely on them as a politician to build roads, for example. This happened to the Labour government during the 60s and 70s. The state bureaucracy violently put down the Allende government and the Bosch government as well.

Finally, it accustoms the proletariat to place its fate in other people's hands. It disengages people from direct action.