Here is a possible explanation; you can find supporting details here: Most of the materials are in the form of translated memoirs of Bismarck and Wilhelm II. While, obviously, biased, the two agree on principal facts.
From the editorial comments:
… Clearly, the Anti-Socialist Laws of 1878 had failed to curb the growth of militant labor organizations. Bismarck aimed at confrontation, hoping that if the laws were allowed to lapse and workers went too far, he would be able to revise the Constitution of 1867-71 in an undemocratic fashion. Wilhelm, at least early in his reign, wanted to win the love of the workers, wooing them away from Marxist socialism with concessions...
The upshot of the story is that Wilhelm II was in favor of repealing the Anti-Socialist laws. (Here he went against the opinion of Bismarck and his cabinet.) As for Deutschkonservative (the voice of the semi-authoritarian kaiser), they were staunchly monarchist (more so than, say, the Freikonservative Partei) and, hence, (I am guessing here), would not go against the clear wishes of the Kaiser.
As for the Nationalliberale Partei (NLP), they actively contributed to the repeal of the Sozialistengesetze.
Here is what Bismarck writes on the NLP role in the matter:
… The imminent close of the Reichstag session raised the question of a renewal of the [Anti-Socialist Laws], which would otherwise expire in the autumn. In the commission, in which the National Liberals struck the first blow, the authority to banish was expunged from the proposal of the Bundesrat;  consequently the question was raised whether the confederate governments would comply in this particular or whether they would wish to retain the power of banishment because of the danger that the bill might not be passed. To my surprise, and in contravention of my strict instructions to him, Herr von Bötticher proposed to introduce on the following day, when the last sitting of the Reichstag would take place, an imperial proclamation by which the projected bill would be revised in the sense desired by the National Liberals –– that is, the power of banishment would be voluntarily renounced –– which could not be accomplished in a constitutional manner without previous consent of the Bundesrat …