A lot of king snakes tend to be pretty striking (that milk snake is also a king snake).
They also tend to be very docile and generally easy to care for, therefore common pets.
I've owned all kinds of snakes and my mexican king snake (shiny black with white specs) was one of my favorites. He liked to go in the aquarium. He'd sink under the water and sit with his tail wrapped around a decoration and watch the fish. Never tried to eat any though.