People are downloading the poses, and not the Sims, that is the fact. Most often, what you see in a custom pose doesn't match the Sim in the gameplay.
(But some Sims with custom poses do really match the picture, of course, and they are awesome.)
I tried with a custom pose for the first time few days ago, and just made my Sim looking like a whore with a pose who didn't manage to sell herself. Now she has whole 4 downloads from my Origin friends, but with Newest as the default search, I bet she would have had at least couple of dozen downloads so far, and she would have made into the Popular Now category.