I have read many different takes on this question. A decent if incomplete explanation is that these forces were (a) demoralized and (b) split against each other.
(B) is the less complex explanation, so I'll briefly address this. To the KPD, the SPD was a "fascist" political party that had ruined the promise of the 1918 revolution. In 1932, the KPD joined the Nazis in a transit strike in Berlin. In other words, the KPD had no interest in maintaining democratic institutions. We can assume, also, that the Reichswehr would have moved with alacrity against any uprising on the part of the KPD or associated militias.
(a) Requires us to backtrack to the events of summer 1932. Franz von Papen is Chancellor. Eager to break the power of the SPD, he deposes the SPD government of Prussia by force. The Prussian state government was the last bastion of SPD power. Prussian Premier Otto Braun and Minister of the Interior Carl Severing, whose powers extended over Prussia's large and well-armed police force, would have been the ones in a position of sufficient authority to call a strike together. But contrast their position to that of Friedrich Ebert and Gustav Noske when these men called their strike in 1920. They were, respectively, President and Minister of Defense. They spoke with the authority of the nation in some sense, while those launching the coup were of the widely hated and discredited forces of monarchy and militarism. And while much of the military was on the fence in 1920, Reichswehr leader von Seect was, for instance, unwilling to take a positive move before an outcome was decided. In 1932, on the other hand, the Reichswehr stood with Minister of Defense Kurt von Schleicher. An outright strike could well have precipitated a violent reaction from the national government, and may indeed have reinforced their narrative of Communist/Socialist troublemaking requiring more authoritarian government. In sum - Unlike in 1920, Braun and Severing, and by extension the SPD, were in the position of illegitimacy. As Erich Eyck writes, "large numbers of Germans, many of them quite influential, were jubilant at the prospect of getting rid of the Socialists and, if possible, of the unions as well." Finally, if the SPD had poor prospects of launching such a strike in 1932 their prospects were even more grim in 1933, after months of electoral drubbing and the loss of almost every position of power they had once held.
Yet, if they had truly believed in the Republic, wasn't it worth a last ditch effort? Probably not. It is doubtful that the mass of workers could have been called to an effective strike. Not only had many millions of them defected to the KPD and NSDAP, but the unemployment rate would have made such a tactic ineffective. Eych observes: "For how could the trade unions call the workers from their posts when they knew that millions of unemployed were waiting the moment when these places might become vacant?"
A final point. I wrote [here] (https://www.reddit.com/r/AskHistorians/comments/4gqtjr/what_was_the_significance_of_the_new_political/) several weeks ago that Schleicher reached several labor union ministers and persuaded them that they would be granted positions of power in the new order, thus keeping them from launching a coup. I have to admit, I may have been seduced by good story-telling. While it's plausible, I haven't seen the evidence for it.
Cited:
Erich Eych, History of the Weimar Republic, Vol. II John Wheeler-Bennett, Nemesis of Power
The Comintern followed a policy of forcing splits inside socialist parties up until 1934. Parties would split into socialists (trying to work in the parlamentary system) and communists (following a revolutionary way to power). Nazi rule was expected to be short and cause an uproar that would allow communists to come to power. In this context, the KPD didn't put up any resistance as the NSDAP was climbing to power, even assisting them in certain situations. As the NSDAP managed to hold on to power and crack down on the opposition a lot more effective than was expected the Comintern changed its policy and in 1934 started propagating the idea of "popular fronts" in the countries not yet under fascist rule. For instance, the Blum Popular front government in France, a wide coalition of parties intended to oppose the spread of fascism. To achieve this, the Comintern and the communist parties eased up on the whole "revolutionary" rhetoric.