Wikia allows users to create their own wikis. However, the site wants all new wikis to have broad topic scopes and be able to attract large communities of editors. Wikia enforces this policy by having a submission process that weeds out wikis with too narrow topics.
If your new wiki topic is too narrow, you can instead add your topic as a subsection under an existing wiki or create a mini-wiki in the Scratchpad Wiki Lab.
