What factors allowed music to thrive during the 1700s?

In a single word: the Enlightenment.

Explanation:

The process of musical secularization, which means a song that is not related to religion or supported by religion, began in the Renaissance but was just with the Enlightenment and the anthropological perspective that music could thrive. Theaters, saloons, royal balls, and other locations were now where music could be heard and appreciate, and not only in the Church.

The classical period is the period of Western classical music between the second half of the eighteenth century and the early nineteenth century, characterized by clarity, symmetry and balance. The best known composers of the period are Joseph Haydn (1732-1809), Wolfgang Amadeus Mozart (1756-1791) and Ludwig van Beethoven (1770-1827).