Final answer:
The Marquis hasn't told the story before now due to his desire to protect other slaves from the risks of escape.
Step-by-step explanation:
The reason the Marquis hasn't told the story before now is because he has certain reasons for keeping it to himself. The evidence to support this can be found in the text where the Marquis states that he would rather suffer under imputations and not exculpate himself. By suppressing important facts, he is protecting other slaves from the risks associated with escape.