For one thing, it's against DOJ policy to bring charges in a case with political ramifications 50 days before an election. For another, as you may recall, Judge Aileen "Loose" Cannon has appointed a special master to review the documents. While an appeals court panel rightly issued the judicial version of a rebuke for her denial to the Justice Dept of access to classified documents, there are lots of others to review. And keep in mind that it's a crime to take any any documents from the White House without NARA approval. So the documents that are still being reviewed may also be relevant to the investigation.