The seeds of the Renaissance period were planted during the crusades. At this point, the Europeans invasion of the Holy Land brought them into contact with spices and other trade good material that they had never seen before. Also, they saw a nation/culture with a more advanced mathematical system as well as a greater knowledge of anatomy and medicine.
Unfortunately, the Black Death nipped in the bud any immediate benefit this might have had to the european subcontinent (2/3 of your population getting wiped out stymies a lot of things).
After the Black Death passed, you quickly (historically speaking) arrived at the Birth of the Renaissance, generally held to have started around 1450 in Florence, Italy. One of the biggest results that came about was the rise of the mercantile class. Before this time, there was no mercantile class. You had people doing that work, but as a socio-economic class, they didn't exist. With trade and commerce expanding greatly, you not only had a rediscovery of the arts (painting, literature, music, etc.) and old technology rediscovered, but also a willingness of those with money to act as patrons to would-be scholars, artists and scientists.
Now, going back to the core books, the authors have made one assumption about your reading: Greyhawk. Although never presented as a full-fledged setting in the books themselves, they've stated many times that Greyhawk is the "default" setting that the rules are trying to portray.
How medieval do you think Greyhawk is?
Unfortunately, there have been no publications for Greyhawk since early 2E (except for Living Greyhawk and I don't know how much info that gave or even if it's still around. I don't think it is; can anyone confirm this?) so many gamers don't know squat about it.
jolt