1) They have become objects of desire with wealthy patrons who have "gotta catch 'em all".
2) They glow in the dark. Enchanting as it is to humans, faeries find their bioluminescence an annoyance. As a creature with many natural predators lighting up at night is akin to wearing a neon sign that says 'Eat Me'.
3) They are also the key ingredient in popular potions and liquid spells. While their magical properties are still moot among some apothecaries and alchemists, it can't be argued that the Purple Cotton Faery can be made into a rather delightful jam.