As you probably know that users contact staff using the contact form as that is where it is meant to go. As some people know the request gets rejected. So I will tell you why it some times get rejected and some times why it gets approved.
So what does "Closing A Wiki" mean? Closing a wiki means that the wiki is deleted and can never be recovered if you haven't requested it to be open again in about 3 days maximum. You can request a wiki to be open and closed through the Contact Form.
Reasons why wikis are closed.
The wiki may be a Spam wiki- This means that the wiki is/was created to be a wiki full of vandalism and fake information.
The wiki may have a few pages- If the pages have about 3 pages only then staff may close the wiki.
The wiki may be inactive- This means that no users have participated on the wiki for more than 2 months.
The request is made by the founder and the wiki may be new- If the founder requests the wiki to be closed then it should be closed.
The wiki is inappropriate by that I mean things that are rude. For example nudity, pornography and racism.
Why wikis are kept open.
They are community owned projects and a wiki you no longer want to participate on can wait for someone else. If you are an admin on the wiki and you don't want to participate on the wiki then you can remove your rights through Special:UserRights. If there are a few active contributers on the wiki then if you are a bureaucrat then you can give the other user admin and bureucrat rights before removing yours.