Because they’re hungry for protein. That’s why honey bees eat chicken feed. Especially in the early spring when the queen is laying again and there are more mouths to feed. (Spring is a relative term for beekeepers on the island of Newfoundland.)
I gave my bees pollen patties earlier in the winter and they showed little interest in them. But judging by how intensely they’re digging into the chicken feed (full of protein), I bet they wouldn’t say no to a protein-rich pollen patty right about now.
April 2019 Postscript: Honey bees are often starving for protein in the early spring before much of anything has flowered. They will dig into anything that looks or smells like protein, including chicken food, dog food, bird feed, a pile of sawdust — you name it, they’ll go for it.