They were outnumbered three-to-one by the SA and other right-wing paramilitaries, not to mention the Reichswehr. If they do, they will be easily crushed. Probably the opportunity to have the Social Democrats call upon the Reichsbanner was a year earlier in the Prussian coup; an armed uprising would have been crushed then as well, but Richard Evans theorises that the right's fear of a civil war would likely force them to back down had it (almost) come to that. But as he says, it is unlikely it would happen to begin with.