Gayre in hist1 almost has a heart attack every other page when he talks about tastes concerning mead. Recipes depend on supplies: if grape is cheap some is added to the mead, if malt is cheap it is used... But recipes also depend on the tastes of customers of that time.
In Viking mythology, the wife of the god Thor gives some aged mead to the god Loki. So Vikings considered aging as improving mead, as gods drank it aged. It must still have been drunk young (at the end of or during the fermentation), that is still sweet.
From the XVIth to the XIXth centuries dessert wines are fashionable at the English court. Wine was imported from Canary or from Crete. Porto, Sherry, Madeira were also imported in the XVIIIth and XIXth centuries. So it is likely that mead had to fit the mainstream tastes and was made sweet. Gayre says that queen Elizabeth even added some sugar to sweet wines.
From the Middle Ages, mead is so expensive compared to wine and beer that only rich nobles could buy some. But not only they could drink mead. Peasants drank some they made themselves. Beekeeping techniques made the extraction of honey difficult: at the end remained a mixture of honey and wax that could not be sold. It was mixed with water and fermented. Gayre states that peasantry, having scarcely educated a palate, preferred sweetness. It is true that children for instance prefer sweet food. But Gayre's views are obviously too condescending: the food of the rednecks in many countries is much better than the one of the court of his country. Additives (spices, herbs) were required to make this syrupy "mead" drinkable. It was doubtlessly drunk young. Not precisely the drink of the gods.
As mead was much more expensive than table wine, its sale was complicated. But as sweet wines were imported from Spain, Italy or even Crete, they were more expensive than (French) dry wines. So the marketing must have made mead makers focus on this niche were they could be competitive. Unlike wine, mead can easily be made dry or sweet at will.