The subject choice is great, I love the detail in the shot. Lighting is also great, with the light highlighting the needles. Perhaps a different composition would have been good, would it have been possible to find a lone cacti and then centred it in the frame, with the top set off by the sky? it would have also been a good macro subject, getting in close and showing the symmetry in the detail of the cacti. This is a great shot, bur perhaps suffered in the voting through the lack of obvious symmetry. Good Luck in the next challenge.
I like this particular cacti for this challenge. It has great lines. It is a good image. I would have tried taking center one (or perhaps one of the other nearby stalks) and centered the tip top of the cactus EXACTLY on the top center of the frame to emphasize symmetry.