Board Thread:Support Requests - Getting Started/@comment-4056542-20130106164908/@comment-3335785-20130106180236

Adverts cannot be removed as outlined in Wikia's terms of use. Website hosting costs money, and it has to come from somewhere. Unfortunately, Wikia use some of the most obnoxious and in-your-face adverts you will come across on the internet.

As for why some wikis don't have adverts, I would imagine they are only monetised when the wiki reaches a sufficient level of activity/size. My test wiki is lacking adverts because of this.