Why did Charles IX not marry Mary after his brother's death? If the marriage was advantageous for France when Francis was king, would it not be for Charles as well? She was Queen of Scotland with a claim to the English throne, as well as being his brother's widow -- both seem legitimate reasons to marry her. Some sources I've seen question whether Mary and Francis even consummated their marriage, so it seems like lack of virginity wasn't the issue. I've done some reading and can't find anything to indicate this match was even discussed. Does anyone know why?
Charles IX inherited the throne when he nine and so a regency was put in place. His regent was his mother, Catherine de Medici. By all accounts Catherine disliked Mary and soon after Francis II's death made Mary return home to her native Scotland.
Catherine likely didn't like Mary due to her Guise heritage. The House of Guise were a powerful French family that had claims to the French throne. Mary Stuart's mother was Mary Guise and her uncles were leaders of the Catholic League which sought to persecute French Huguenots. This was in direct opposition to Catherine de Medici's somewhat more tolerant policy.
Considering the Guise influence already in France and their policies of persecution fanning the flames of religious war in France, it would have been unwise to increase Guise influence in France by marrying an eighteen year old Mary Stuart, the daughter of a Guise, to the child King of France. This would have given the Guise's absolute influence on the French Crown and reducing Catherine's own influence as the current regent and left her vulnerable.
Besides this, in the absence of their monarch, by 1560, Scotland had seen the Reformation take hold and the 1560 Treaty of Edinburgh had basically seen Scotland switch from a pro-French stance to a pro-English one. This effectively ended the anti-English Auld Alliance between Scotland and France, raising the question of how useful a marriage alliance with Scotland was to France - history would show not very as the circumstance in Scotland (as well as poor choice on Mary's part) would mean Mary wasn't Queen of Scots for long after her return.