In school, I was told that religious houses produced mead almost as a by-product because they kept bees to make wax for altar-candles, and the easiest way to separate the wax was to wash the combs. Consequently they ended up with loads of diluted honey which will pretty much turn into mead by itself if you ignore it. Is this true? I can find plenty of corroboration for the acquisition-of-wax part of this but none for the practice of washing out the honey.
Mead was never a high volume product at any time during the middle ages (which I assume is the time frame you are inquiring about). Even in places where mead was quite popular such as Poland, Scandinavia and Wales it was a special occasion luxury good. In warmer climates religious houses primary made wine and in cooler areas beer/ale and to a lesser extent cyder. Mead was produced but not to the extent of the other beverages listed.