Why is wheat not used in lager?
Before the Reinheitsgebot, the Munich city council decided that it would try to control brewing quality, and drafted laws that specified beer to only be made with barley, hops and water, not mentioning yeast. Prior to this, and before refrigeration, German beer made outside of winter was warm fermented, typically taking place in open vats that were spontaneously fermented. The source of fermentation was not well understood at the time, which explains how yeast went ignored in the original purity laws. The warm fermented beer made during the summer was inconsistent and did not keep as well as the bottom fermented beer made during the winter months. Summer brewing was even outlawed, limiting the spread of these ales. However, according to the Reinheitsgebot, the one style that was allowed to be brewed in the summer was wheat beer, and the specifics of the Reinheitsgebot allowed for wheat only as an ingredient in top fermented ale, not in lager. Hence bottom-fermented weizenbier never gained cultural footing in Germany, and to this day we still largely see all barley German styles throughout the world, with wheat still used almost exclusively in top-fermented Weizenbier.
Before the Reinheitsgebot was enacted in 1516, the brewing industry in Bavaria was buying up wheat in order to make beer, leaving other grains, such as spelt, oat, and barley, to the bakeries. The bakers and the population in general felt that bread made with these grains was less enjoyable and thus less in demand, and William IV, Duke of Bavaria, heard enough complaints that he addressed the problem in crafting the Reinheitsgebot. Beer making was to be limited to barley only, with the one exception allowing wheat in warm fermented summer beer.