Monarch butterflies lay their eggs on milkweed, and as young caterpillars, they eat the stuff. But humans like to have neat rows of corn and soybeans, and milkweed interferes with their field aesthetics. Which is more important? Doesn’t matter; humans have thumbs, agriculture, and industrial chemistry. Thus, 100 million acres of row crops are now milkweed-free; Monarch butterflies have fewer places to stash their young; and their population may be dwindling.
Because farmers don't like milkweed, but everyone likes butterflies, insect experts are advocating for milkweed preservation efforts on conserved farmland or in personal gardens. This seems like a hard sell, as generally people think of "weeds" as something you get rid of, not make a particular effort to grow. Perhaps milkweed needs a rebranding campaign; something like "pretty butterfly kiss bush" or "if you kill this plant you're killing butterflies plant" should do the trick.