Why did extremist parties fail to win many votes during the Weimar Republic

Why did the extremist parties fail to win votes during the Weimar Republic years?

The Weimar Republic managed to survive many political and economic crisis and this was essentially because there was no viable alternative at this stage. The reasons can be summarised as follows:

1) Extremists cancelled each other out! - Neither the extreme left or extreme right could get the upper-hand over their rivals. The Spartacist Rising was defeated brutally by the Freikorps and the Kapp Putsch was defeated by Berlin workers who went on strike.

2) Violence was localised…