Why did the Renaissance start in Florence?

Wikipedia contains articles on both the "Renaissance" that began more or less in Florence and spanned the 14th through 17th centuries, and on the "Renaissance of the 12th centry," describing the 12th century Renaissance. The most significant factor that Objectivism identifies in regard to the Renaissance is the following, from the 12th century Renaissance article (link):

Thomas Aquinas
Main article: Saint Thomas Aquinas

The most famous of the scholastic practitioners was Thomas Aquinas [1225-1274] (later declared a Doctor of the Church), who led the move away from the Platonic and Augustinian and towards the Aristotelian. Using the scholastic method, Aquinas developed a philosophy of mind by writing that the mind was at birth a tabula rasa ("blank slate") that was given the ability to think and recognize forms and ideas through his intellectual powers that were a participation in the divine intellect.

Aquinas is most well-known for having composed the Summa Theologiæ, which has been analogized to the Gothic cathedral: as the Gothic cathedral is the expression of mediaeval architecture par excellence, the Summa Theologiæ is the expression of mediaeval intellect and philosophy par excellence. The Summa Theologiæ is considered to be the pinnacle of scholastic, mediaeval, and Christian philosophy as well as Christian and Catholic theology. He also wrote the lesser-known Summa contra Gentiles, an early missionary's manual that covers much of the same material; many treatises on problems theological and philosophical; and a series of commentaries covering most of the Aristotelian corpus.

Objectivist scholars and others probably would include both Renaissances as simply different phases of a single historical development in the transition from the Medieval-Christian era to the "Renaissance-Modern" era (including the Enlightenment), whose influence still persists today (culturally if not necessarily throughout academia).

Objectivism attirbutes the cause of this transition to (a) the rediscovery (in Constantinople) of numerous long lost works of Aristotle as a result of the Christian-inspired drive to explore other lands; and especially (b) the honesty and expert scholarship with which Aquinas brought Aristotle's ides to the attention of so many others, who further spread those ideas gradually throughout the whole Western culture. In fact, Leonard Peikoff once proposed Aquinas as the "Man of the Millennium" for his influence in bringing Aristotle's ideas back into the world (reserving Ayn Rand for the next millennium).

Regarding the role of Constantinople, the Wikipedia article on "Constantinople" observes:

It was especially important for preserving in its libraries manuscripts of Greek and Latin authors throughout a period when instability and disorder caused their mass-destruction in western Europe and north Africa: On the city's fall, thousands of these were brought by refugees to Italy, and played a key part in stimulating the Renaissance, and the transition to the modern world. The cumulative influence of the city on the west, over the many centuries of its existence, is incalculable. In terms of technology, art and culture, as well as sheer size, Constantinople was without parallel anywhere in Europe for a thousand years.

Regarding the article by Mary Ann Sures in The Objectivist, it must be remembered that Mrs. Sures is primarily an art historian, and her article was primarily concerned with the Renaissance in art. It takes time for new ideas in the more fundamental branches of philosophy to spread gradually as deeply into esthetics as they did during the Renaissance.