The simple answer is, Google. Google ranks websites based on all sorts of factors, such as the amount of unique content your site has, how often it's updated and what types of images you share.
The more nudity, the lower down we rank. Basically, Google doesn't like nipple!
Here is a great blog by our director of Ecommerce (aka in-house internet wizard) Matty Curry, talking about Google, rankings, and nipples!