Because nihilists seek the destruction of everything that comprises society, and because that aspiration will never be shared by a majority (or even a substantial portion) of the population, to wait for mass consensus is tantamount to defeat.
A different cell of the FAI writes: "We don’t even give a minute of our life in the hope that the multitude will suddenly become aware and wake up! If the oppressed are not ready to raise the hatchet, this is a problem of the oppressed."