These challenges need good advertisement on the home page. I think thats the main reason why there are not so many entries in these challenges. I also notice that this site brings in alot of people that are just breaking into 3D modeling and ect.., so skill level is also a major factor in my opinion, but it also seems like the ones that are skilled are too busy to enter; but however, there were a few entries in this challenge that had a very skilled artist behind it. Sadly they just dropped out, be it for personal reasons, or for some other reason.

Of course this is just my own personal opinion.