I get this question a lot, and, to answer, I will do a David Letterman-style Top 10 "Reasons Why Sake Was Made In The Winter" List:
10. Rice was planted in the spring and harvested in the fall.
9. Rice needs to sit for a month after harvesting to "acclimate".
8. After polishing rice the milled rice needs to "acclimate" again.
7. Sake brewers who are called Kuribito were really farmers who had nothing to do during the winter months.
6. As fermentation creates a great deal of heat, outside air was used to cool down the process naturally.
5. Koji rooms also get very hot and the winter air provided a cool and "local flavor" blast of air.
4. Storage facilities were sometimes made of snow.
3. Natural airborne yeasts are practically eliminated in cold temperatures.
2. Some brewers prefer the flavor of winter water to summer water.
1. Sake and snow just go together!