The short answer is that we are not told why. Within the context of the book of Job, God uses Satan's evil for the ultimate good of vindicating Job's faith, multiplying his blessings, and glorifying God's name. God demonstrates intentionality with Satan's presence. Just as Job never is told about the spiritual backstory to his suffering, we are not told about the backstory to Satan's presence. This is an intentional aspect of the book. As a book, one of the main lessons is that God does not always explain suffering and that human explanations are lacking and even exercises in folly when they are not careful to be in line with God's revelation. Likewise, we are not given answers to all the questions that we as the audience have. We have to take what God will give us, and that is a story where Satan is granted the opportunity to present his case before God.
One important distinction also is that there are some who, in their preface to the gospel, will say things like, "God is too holy to be around sin" is the explanation for why people need salvation. This is not exactly true. God is fully able to be in the presence of Satan and the entire fallen creation is in his dominion. His holiness is his separateness and otherliness from creation and especially fallen creation, but not some kind of inability on his part. He is righteously angered by sin and does exact justice, but it is not true that he has some kind of inability to be in Satan's presence. God will ultimately destroy Satan, but within the context of Job, he allows him to present his case, the case that ultimately backfires.