It's the dirt.
"Another big issue in recycling agricultural plastics is dirt and debris."Another big issue? How about it being the biggest issue. I was at a plastics recycling conference a few years ago and all the speakers kept talking about was the dirt. Dirt is going to contaminate the plastic and discolor it (unless it is already black). Consumers are all in favor of companies using recycled plastics and some are even willing to pay a little more for it, but no one is willing to give up on appearance. White plastic with dark brown specks, line and swirls? Not too likely.
At that conference, one of the speakers showed prototypes how using the unwashed plastic could generate tiles for use in plastic sidewalks. The appearance was nice, as the nonuniform discoloration provided visual interest, much like slate, marble, granite and other natural materials do. I've not heard much about that application since, but it was mentioned in passing in this article. Sadly, there aren't too many applications like that that can take in the plastic "as is" and run with it. The plastic can be washed, but then you have to dry it afterwards. Water and molten plastics just don't get along.