[Cover art of the 1508 edition. A knight on horseback became the standard cover art for chivalric novels.]
Gutenberg's printing press became operational in about 1450, and in 1455 he published 180 copies of the Bible. Though these were expensive, about three years of wages for a middle-class salary, they were considerably cheaper than hand-copied books; it took about a year to copy one Bible by hand.
Soon, inexpensive paper replaced parchment, and printing spread. By 1500 there were 1,700 printing presses in over 250 locations; they had cumulatively published at least 15 million copies of thousands of titles. Almost half dealt with religion; many of the rest served scholarly or scientific uses, with topics like grammar or medicine, or were reprints of Classical texts. Three-fourths of the titles were in Latin. But books in the vernacular for entertainment sold well, too.
Education and a rising incomes had expanded literacy rates, and reading, sometimes even for pleasure, had become common among the nobility and the middle and upper classes. Those who could not read often heard books: reading out loud to groups became a popular pastime, sort of like going to movies today.
In Spain, 35 editions of chivalric novels were published in the 1530s; 49 in the 1540s; 20 in the 1550s; 29 in the 1560s; 7 in the 1570s (during an economic crisis); 31 in the 1580s; and 4 in the 1590s — this despite frequent censure of these books as "frivolities and proven lies." The print run of an edition might typically be 1000 copies. Amadís de Gaula itself was reprinted 20 times during the 16th century within Spain and had 8 sequels.
In addition, many copies were printed outside of Spain in Spanish. In Italy, Germany, France, England, and Portugal, Amadís and its sequels were translated into the local language, and the stories were continued by other writers — 24 volumes alone in France by various authors.
In those days, one copy did not equal one reader. Books were expensive, so they typically were lent, rented, and read in groups. So while the number of books may seem small by today's standards, each book was reread repeatedly over the years, and the overall population of Europe was smaller, too.
There's a theory that every best-seller is an accident. If publishers knew for sure how to create them, every book would be a best-seller. Why was Amadís the first to sell so well and launch a genre?
In 1500, the time was ripe for some book to become generally popular, simply because printing and reading had become widespread. Spain had also become the leading world power, and people wanted to learn Spanish. Reading literary works is still a good way to study a foreign language.
But of course, there's something else, something that excited readers. Brave knights, beloved damsels, and fantastic adventures captured the popular imagination during the Renaissance for some reason.
It may have been the deep cultural roots that chivalric stories had all across Europe. Or the novels may have reinforced the sense of individual possibility and personal achievement at the same time that renewed scholarship, exploration, and discoveries expanded the readers' real-life horizons. But the Renaissance was also a time of political and religious conflicts and frightening change, so readers may have appreciated a retreat to simpler times. Or maybe the novels seemed entertaining, and we all do things just because we enjoy them.