I'm actually an architecture major, so maybe a historian can complement my answer, but the techniques used in the architecture of high and late Middle Ages made the use of glass windows and panels much more common, specially on ecclesiastical buildings.

One of Gothic's (roughly 12-16th century) main characteristic is the use of what is known as flying buttress, a structure that supports and transfers the weight of the walls and ceiling to the ground, allowing these walls to be thinner and, therefore, the use of glass without compromising the building's structure.

http://en.m.wikipedia.org/wiki/Flying_buttress http://en.m.wikipedia.org/wiki/Gothic_architecture

Edit: grammar

Glass was expensive into the mid-late 1700s and plate glass is developed in the 1800s and becomes affordable for common household use in the last half of the century. Before that, shutters and translucent oiled skins served as an alternative for light and temperature control but often the openings were left open as the climate allowed.