The young Louis XV was in a very precarious situation dynastically. Both his parents and his brothers had died young due to illness and the king's health was fragile. So, a bride was vital for carrying on the French throne. Initially, an agreement had been reached with the Spanish king that Louis XV would marry Infanta Maria Anna Victoria.
The Infanta was very young, though - she was only three years old when she arrived at Versailles while Louis was eleven. She remained at Versailles for a couple of years until something happened to drastically change the situation. Louis XV's health was delicate and he became ill which prompted the Prime Minister - the Duc de Bourbon - to annul the agreement and ship the Infanta back.
The search was on for a bride who was healthy and old enough to carry an heir. A list was made of 99 princesses who would all suit as a potential queen of France. The list included the names of the princesses, their dynastic heritage, their ages (ranging from 13 to 22), their appearances, health etc. Some of these were:
Born: 29 December 1709
Reason for rejection: Peter the Great - her father - visited Versailles and Louis XV while the king was still a boy. While in France he suggested his daughter, Elizabeth, as a bride for Louis XV. However, she was initially rejected because of her mother who had been a maid.
Became: Tsarina of Russia
Princess Anne of England
Born: 2 November 1709
Reason for denial: Her father - George II of England - was primarily on the throne due to being a Protestant. If Anne was to become queen of France she would have to convert which was rejected.
Became: Princess of Orange
Princess Anne Charlotte of Lorraine
Born: 17 May 1714
Reason for rejection: the House of Lorraine was a direct rival of both the House of Orléans (which the Regent belonged to) and of the Duc de Bourbon
Became: Abbess of Remiremont
Landgravine Caroline of Hesse-Rotenburg
Born: 18 August 1714
Reason for rejection: she was infamous for her bad temper!
Became: Duchesse de Bourbon (meaning that she married the Duc de Bourbon instead!)
Henriette-Louise de Bourbon, Mademoiselle de Vermandois
Born: 15 January 1703
Dynasty: Bourbon (she was the sister of the Duc de Bourbon)
Reason for rejection: she refused to marry the king since she would rather join a convent
Became: Abbess of Beaumont-lès-Tours
Princess Amelia of England
Born: 10 June 1711
Reason for rejection: like her sister, Anne, she was a Protestant
Became: remained unmarried as Princess of England
Élisabeth-Alexandrine, Mademoiselle de Sens
Born: 5 September 1705
Dynasty: Bourbon (another sister of the Duc de Bourbon)
Reason for rejection: not quite clear but possibly doing to her close relationship with her brother. Cardinal Fleury was eager that the Duc should not get more power.
Became: remained unmarried with the title of Mademoiselle de Sens
Princess Barbara of Portugal
Born: 4 December 1711
Reason for rejection: unknown - possibly it was considered too much of an insult to Spain
Became: Queen of Spain
Princess Charlotte Amalie of Denmark-Norway
Born: 6 October 1706
Reason for rejection: Denmark-Norway's tense relationship with Sweden, an ally of France
Became: remained unmarried as Princess of Denmark-Norway
First, the list was shortened to 17 princesses and then further reduced to four - including both sister of the Duc de Bourbon. However, Louis XV and Cardinal Fleury rejected them all and the original list had to be looked at again. Surprisingly, Marie Leszczynska had not even made it to the list of 17 candidates. The choice was considered an odd one: an impoverished second daughter of a man who had lost his throne. Add to that that she was already 22 years old.
Nevertheless, politically it was a good choice. After the snubbing of the Infanta France had to be careful not to antagonize their neighbor further - the worst-case would be the eruption of a war. The choice was diplomatic nightmare. The English could not accommodate the conversion to Catholicism, an Austrian would upset the Spanish while a Dane would definitely create a drift between France and Sweden - as would a Russian potentially.
Marie Leszczynska had the clear advantage of being a politically neutral choice. Since her father was no longer king of Poland he was not considered to be a danger anymore and therefore his daughter would insult few monarchs. The young woman herself was old enough to provide an heir immediately and while not considered beautiful she was warm, kind and devout.