The striking thing was that France and Austria had been political rivals going back to the time of Francis I (France) and Charles V (Austria). Until the mid 18th century.
After winning the 100 Years' War, France became the strongest power in western Europe. Spain and Austria (counting the Holy Roman Empire) were two and three, and when Princess Juana of Spain married Prince Philip of Austria to produce (Holy Roman) Emperor Charles V, the combination of the "next two" became stronger than number one.
Fast forward to the 18th century. After two centuries of mutual antagonism, France and Austria were pushed into an uneasy alliance against a combination of two new "upstart" powers. England had about one-third the population of France, and Prussia about one third of the population of Austria-Hungary, but both of them "punched above their weight," to the point of "stalemating" France and Austria-Hungary (Russia switched from the Franco-Austrian to Anglo-Prussian side at the end of the Seven Years' War.)
The marriage of Austria's Marie Antoinette and France's Louis XVI cemented the alliance of two "legitimate" great powers against the two "upstarts."
It's also noteworthy that Maria Theresa herself was married to Francis, Duke of Lorraine (then a part of France). If the Austrian Court was willing to marry the heiress to the throne to a French duke, it certainly wouldn't object to a non-heiress princess marrying a Dauphin.