I have lived in Washington 23 years, and I have certainly never seen anything like 86 varieties of butterflies. More like three, and one was probably a moth. Not at all. So in the city you see only those types that eat the kinds of plants allowed to grow there, which are kind of limited. We are talking about weeds.
Butterflies love to hang out on the edges of fields and pastures, where the weeds grow happily.
In a city, a field is not a field but a vacant lot and is apt to be paved over or mowed. But to butterflies, those independent plants, from clover to dandelions, from crabgrass to viper's bugloss, are gourmet-quality foods. Most butterflies are very host-specific. They like just one or two kinds of food. The monarch, for instance, the famous big orange creature with black borders that looks like a Tiffany lamp, loves milkweed in its larval stage.