As others have said it is the low spot in the cab so everything ends up there, bugs, dirt, bits of flotsam and jetsam. I coated mine inside with POR-15 during and after final welding. They will never wear out in my lifetime but if your replacing them you want to do it right. Coat and allow a drain hole to blow stuff out on occasion.
http://www.flickr.com/photos/advance-design_parts_co/8129083489/in/set-72157631867396207