Mark, with his usual severity, makes Jesus angry and disappointed, and also turns the insiders into outsiders. They cannot answer this riddle, any more than they could read the Parable of the Sower.
Most of us would think of Mark as the conveyor of the parable for us to interpret, however, this article makes it seem more like everyone that wasn't Jesus had to first interpret the parables themselves before talking about them and writing them down. The interpretations conducted by Mark and Matthew were unavoidably influential on their writing method--they intruded on the parable. Thus, we are left with an already partially interpreted parable instead of the original.