Why are bees attracted to Stag Hides in Lime bath
Short answer: They are probably looking for minerals from the lime

Explanation:
Insects from time to time need minerals (salt) they can't get from there floral diet. Because they can't chew these directly of rocks, they suck these up from puddles or occasionally from human sweat. This behaviour is well known from butterflies, but also from honeybees

Notice BTW that there are indeed several bee species (Probably Apis mellifera, Bombus terrestris agg. and Bombus lapidarius), but also several flies including Eristalis cf. tenax, a hoverfly.

Why not from the sheep hides? I'm not sure, but the water around the sheep hides looks awfully brown, suggesting there are a lot of humic acids un the water. That might be less attractive to the insects.