My guess would be they don't need/want them short. Sharp, long nails would be ace at cutting through the mud when digging, and this action in itself would, I'm sure, keep them short in the wild.
I guess indoor bunnies would need to have their nails clipped more often that outdoor bunnies, just because of the floor they live on.
I'm so glad that my buns' nails are light- it makes it so much easier to see the quick! With my dogs (they don't need their nails trimmed much at all, perhaps just once a year) they are so dark I just have to try my best- see if they pull away when he start to clip etc. It's so stressful!
Jen