Positives: General technical quality and tonality are the strength of this composition.
Technicals: Tonality is particularly nice in this image, especially the browns on the cacti. They are just plain nice. Sharpness is very good except for a little haloing around the right arm and a bit of digitalization visible in some of the "skeleton" fine detail on my sharpness sensitve LCD monitor. The clouds, which sometimes are really hard to find in Arizona, make a nice background.
The Challenge: In free study challenge voters always have exceptionally high expectations and look mostly for two things. 1-Exceptional technical quality and 2-The elusive "wow" factor. Your technical quality is good but not exceptional and the "wow" factor kinda low. Combined, voters felt yours was a little below average for the challenge.
Suggestions: kawhona makes some excellent observations and very good suggestions in the comments. I'd go with what he says. His comments remind me of this picture similar to yours that I took down in Tucson using a polarizer:
I think the shot is very static and the light really isn't complimentary. If you look around I bet you will find lots of dead ones laying on the ground those can give you some interesting shadows and textures. For this one if you can wait for sunrise/sunset and use the polarizer it will pop alot more. If the sky is kind of bland like this but, the light is cooler or warmer you might get a better result in Sepia. That's my five cents worth, but, you paid less than that for it so it isn't worth five cents :)