He did this because the law required that the accused be summoned to appear before the judge, before the judgement may be passed upon him, and Moses did not wish these men to be punished without a hearing.
Moses did not immediately carry out these instructions, for he tried once again to warn Dathan and Abiram of the punishment impending upon them, but they refused to give heed to Moses, and remained within their tents.
The mouth of hell approached the spot upon which Dathan, Abiram, and their families stood, and the ground under their feet grew so precipitous that they were not able to stand upright, but rolled to the opening and went quickly into the pit.