The history of music publishing is well documented and the two major textbooks (A History of Western Music (Grout/Palisca) and The Cambridge Music Guide (Sadie/Latham)) along with prominent Internet portals (Wikipedia, Petrucci Music Library et.al.) all point to the end of the 15th Century as the time when moderately large-scale score printing and distribution started to take hold. Before this time most distributed musical work was liturgical as the monasteries and clerical class had access to literate staff who could carry out the laborious task of rewriting each copy of a text as required.
Printing with movable type was developed in China in the 11th Century but wasn’t until the mid 15th Century with the work of Johannes Gutenberg that Europe embraced the technology and started to work and discover what it could do for society.
The real breakthrough for the printed score came with the work of Ottaviano Petrucci in the 16th Century who developed an accurate and clear method of printing the stave, notes and words in three passes producing a high quality copy. Other methods were developed (notably that of the Englishman John Rastell), paving the way for a new facet to social life, that of popular and publicly accessible music for the masses (or at least that sector of it that could afford to buy the printed works).
This had a profound effect in that the higher social classes from 16th Century Europe started to learn and play musical instruments purely for the enjoyment of doing it, generally alone or with a small group of friends and rarely for any real audience. This was happening at the same time as other late Renaissance-era scientific and liberal arts such as the astronomy and poetry emerged from the ivory towers of the purely academic, the many facets of artistic life driving forward the idea of leisure (see my page on “What is Pop” here) as an integral part of civilised life.
With this movement, a new market opened up over the next 100 years for known and respected musicians in that they detach themselves from the yoke of patronage (as in the case of Mozart) and make a living as a producer and performer of music in the public arena, with many musicians taking their lead from Beethoven in the late 18th Century. This was the beginning of the era of the public performance (beginning in 1725 with the composer Anne Danican and the Concert Spirituel in Paris) and with the income from performances and published work composers were able to stretch themselves as never before, writing music for larger orchestras as well as public opera and liturgical service, all of which could be printed, distributed and played by an increasingly proletariat audience.