This is a beautiful photo, nice color and simple. You have captured the texture on the rocks nicely too. However, I see no motion blur in this photo. I see a blurry reflection of the rocks on the water. Since there is no motion blur, that could be the reason for your low score. I assume that you were just confused about what exactly motion blur is. Your photo, challenge theme aside, is very good. I think that the thick white border really adds to the image. The contrasting colors between the rocks and water really make the picture POP. I hope this was helpful.
What can possibly classify it as motion? Blur of slopping water at the boulders? It's so timid that the first impression is actually one of stillness, not movement