Because clipless pedals have their place and not all types of riding require them. It's much easier to use platform pedals when riding in a congested area (like a large city for example) then clipless because of the frequent stops that you have to do. Could you do it in clipless? Yes, but why spend most of your ride time clipping and unclipping. It's also easier to use platform pedals when doing slow, family rides as you don't need the benefits that clipless provides. FYI, around 90% of my riding is clipless.
With clipless being so popular ,and well defended here, why in the world would anyone who uses clipless ever return to platform pedals if clipless is so great????